In 1996 David Boim, a Jewish teenager who was both an Israeli citizen and an American citizen, living in Israel, was shot to death by two men at a bus stop near Jerusalem. His parents filed this suit four years later, alleging that his killers had been Hamas gunmen and naming as defendants Muhammad Salah plus three organizations: the Holy Land Foundation for Relief and Development, the American Muslim Society, and the Quranic Literacy Institute. (A fourth, the Islamic Associa*688tion of Palestine-National, appears to be either an alter ego of the American Muslim Society or just an alternative name for it, and need not be discussed separately. There are other defendants as well but they are not involved in the appeals.) The complaint accused the defendants of having provided financial support to Hamas before David Boim’s death and by doing so of having violated 18 U.S.C. § 2333(a), which provides that “any national of the United States injured in his or her person, property, or business by reason of an act of international terrorism, or his or her estate, survivors, or heirs, may sue therefor in any appropriate district court of the United States and shall recover threefold the damages he or she sustains and the cost of the suit, including attorney’s fees.”
The district court denied the defendants’ motion to dismiss the complaint for failure to state a claim, 127 F.Supp.2d 1002 (N.D.Ill.2001); the defendants had argued that providing financial assistance to a terrorist group is not an act of international terrorism and therefore is not within the scope of section 2333. We authorized an interlocutory appeal, 28 U.S.C. § 1292(b), and the panel that heard the appeal affirmed the district court. Boim v. Quranic Literacy Institute, 291 F.3d 1000 (7th Cir.2002). The case then resumed in that court. The court granted summary judgment in favor of the plaintiffs with respect to the liability of the three defendants other than the Quranic Literacy Institute. 340 F.Supp.2d 885 (N.D.Ill.2004). A jury was convened and, after a trial lasting a week, found the Institute — which having filed a statement of “nonparticipation” attended but did not participate in the trial — liable. The jury then assessed damages of $52 million against all the defendants (including the ones not before us) jointly and severally. The amount was then trebled and attorneys’ fees added.
These defendants again appealed, this time from a final judgment. The panel vacated the judgment and directed the district court to redetermine liability. 511 F.3d 707 (7th Cir.2007). Judge Evans agreed with the reversal as to the Holy Land Foundation but otherwise dissented.
The plaintiffs petitioned for rehearing en banc, and the full court granted the petition, primarily to consider the elements of a suit under 18 U.S.C. § 2333 against financial supporters of terrorism. The parties have filed supplemental briefs. A number of amici curiae have weighed in as well, including the Department of Justice, which has taken the side of the plaintiffs.
The first panel opinion rejected the argument that the statute does not impose liability on donors to groups that sponsor or engage in terrorism. The supplemental briefs do not revisit the issue, and at oral argument counsel for Salah and the Holy Land Foundation disclaimed reliance on their former position concerning the liability of donors. But in a letter to the court after oral argument, Salah’s counsel indicated that the disclaimer had been based solely on a belief that the doctrine of law of the case foreclosed any further consideration of the statutory issue in this court. That was a mistake. The full court can revisit any ruling by a panel. All arguments that the defendants have presented in their appeals are open today — and will be open in the Supreme Court. It is better to decide the question than to leave it hanging; why bother to address the elements of a legal claim that may not exist? Before deciding what a plaintiff must prove in order to recover from a donor under section 2333, we should decide whether the statute applies. United States National Bank of Oregon v. Insurance Agents of America, Inc., 508 U.S. *689439, 445-48, 113 S.Ct. 2173, 124 L.Ed.2d 402 (1993).
Section 2333 does not say that someone who assists in an act of international terrorism is liable; that is, it does not mention “secondary” liability, the kind that 18 U.S.C. § 2 creates by imposing criminal liability on “whoever commits an offense against the United States or aids, abets, counsels, commands, induces or procures its commission,” or “willfully causes an act to be done which if directly performed by him or another would be an offense against the United States.” See also 18 U.S.C. § 3 (accessory after the fact). The Supreme Court in Central Bank of Denver, N.A. v. First Interstate Bank of Denver, N.A., 511 U.S. 164, 114 S.Ct. 1439, 128 L.Ed.2d 119 (1994), held that section 10(b) of the Securities and Exchange Act of 1934, which prohibits securities fraud, does not reach aiding and abetting because it makes no reference to secondary liability, the kind of liability that statutes such as 18 U.S.C. §§ 2 and 3 create in criminal cases. The Court discussed the securities laws at length, but nothing in its holding turns on particular features of those laws.
So statutory silence on the subject of secondary liability means there is none; and section 2333(a) authorizes awards of damages to private parties but does not mention aiders and abettors or other secondary actors. Nevertheless the first panel opinion concluded that section 2333 does create secondary liability. It distinguished Central Bank of Denver as having involved an implied private right of action (for it was a private suit, yet section 10(b) does not purport to authorize such suits), while section 2333(a) expressly creates a private right. But as the dissenting Justices in Central Bank of Denver had pointed out, the majority’s holding was not limited to private actions. 511 U.S. at 200, 114 S.Ct. 1439. It encompassed suits by the SEC, which section 10(b) authorizes expressly.
Congress agreed with this understanding of Central Bank of Denver, for the next year it enacted 15 U.S.C. § 78t(e) to allow the SEC in section 10(b) suits to obtain relief against aiders, abettors, and others who facilitate primary violations. Stoneridge Investment Partners, LLC v. Scientific-Atlanta, Inc., — U.S. -, 128 S.Ct. 761, 771-72, 169 L.Ed.2d 627 (2008). The enactment of section 78t(e) would have been pointless had Central Bank of Denver allowed secondary liability to be imposed in suits, such as suits by the SEC under section 10(b), that the statute expressly authorizes. Years later, reaffirming Central Bank of Denver, the Supreme Court repeated that the earlier decision had not been limited to private suits under section 10(b). Stoneridge Investment Partners, LLC v. Scientific-Atlanta, Inc., supra, 128 S.Ct. at 768-69.
The first panel opinion relied on Harris Trust & Savings Bank v. Salomon Smith Barney, Inc., 530 U.S. 238, 120 S.Ct. 2180, 147 L.Ed.2d 187 (2000), an ERISA case involving an application of trust law. Trust law permits trust beneficiaries to maintain actions against third parties who have received trust assets improperly. ERISA not only does not upset this principle of trust law; it authorizes the Secretary of Labor to penalize third parties who “knowing[ly] participat[e]” in a fiduciary’s misconduct. 29 U.S.C. §§ 1106(a), 1132(i)(1)(B). Harris Trust did not cite Central Bank of Denver and did not purport to limit its holding. Stoneridge, decided eight years after Harris Trust, also did not treat Harris Trust as circumscribing Central Bank of Denver—it did not even cite Harris Trust.
To read secondary liability into section 2333(a), moreover, would enlarge the *690federal courts’ extraterritorial jurisdiction. The defendants are accused of promoting terrorist activities abroad. Congress has the power to impose liability for acts that occur abroad but have effects within the United States, F. Hoffmann-La Roche Ltd. v. Empagran S.A., 542 U.S. 155, 165, 124 S.Ct. 2359, 159 L.Ed.2d 226 (2004), but it must make the extraterritorial scope of a statute clear. Small v. United States, 544 U.S. 385, 388-89, 125 S.Ct. 1752, 161 L.Ed.2d 651 (2005); EEOC v. Arabian American Oil Co., 499 U.S. 244, 248, 111 S.Ct. 1227, 113 L.Ed.2d 274 (1991).
The first panel opinion discussed approvingly an alternative and more promising ground for bringing donors to terrorist organizations within the grasp of section 2333. The ground involves a chain of explicit statutory incorporations by reference. The first link in the chain is the statutory definition of “international terrorism” as “activities that ... involve violent acts or acts dangerous to human life that are a violation of the criminal laws of the United States,” that “appear to be intended ... to intimidate or coerce a civilian population” or “affect the conduct of a government by ... assassination,” and that “transcend national boundaries in terms of the means by which they are accomplished” or “the persons they appear intended to intimidate or coerce.” 18 U.S.C. § 2331(1). Section 2331 was enacted as part of the Federal Courts Administration Act of 1992, Pub.L. No. 102-572, § 1003(a)(3), 106 Stat. 4506, 4521. Section 2333 (having been originally enacted in 1990 and repealed for a technical reason the next year) was reenacted in , 1992 as part of that same Federal Courts Administration Act. So the two sections are part of the same statutory scheme and are to be read together. Nicholas J. Perry, “The Numerous Federal Legal Definitions of Terrorism: The Problem of Too Many Grails,” 30 J. Legis. 249, 257 (2004).
Section 2331(l)’s definition of international terrorism (amended in 2001 by the PATRIOT Act, Pub.L. No. 107-56, § 802(a)(1), 115 Stat. 272, 376, but in respects irrelevant to this case) includes not only violent acts but also “acts dangerous to human life that are a violation of the criminal laws of the United States.” Giving money to Hamas, like giving a loaded gun to a child (which also is not a violent act), is an “act dangerous to human life.” And it violates a federal criminal statute enacted in 1994 and thus before the murder of David Boim — 18 U.S.C. § 2339A(a), which provides that “whoever provides material support or resources ..., knowing or intending that they are to be used in preparation for, or in carrying out, a violation of [18 U.S.C. § 2332],” shall be guilty of a federal crime. So we go to 18 U.S.C. § 2332 and discover that it criminalizes the killing (whether classified as homicide, voluntary manslaughter, or involuntary manslaughter), conspiring to kill, or inflicting bodily injury on, any American citizen outside the United States.
By this chain of incorporations by reference (section 2333(a) to section 2331(1) to section 2339A to section 2332), we see that a donation to a terrorist group that targets Americans outside the United States may violate section 2333. Which makes good sense as a counterterrorism measure. Damages are a less effective remedy against terrorists and their organizations than against their financial angels. Terrorist organizations have been sued under section 2333, e.g., Ungar v. Palestine Liberation Organization, 402 F.3d 274 (1st Cir.2005); Biton v. Palestinian Interim Self-Government Authority, 252 F.R.D. 1 (D.D.C.2008); Knox v. Palestine Liberation Organization, 248 F.R.D. 420 (S.D.N.Y.2008), but to collect a damages *691judgment against such an organization, let alone a judgment against the terrorists themselves (if they can even be identified and thus sued), is, as the first panel opinion pointed out, 291 F.3d at 1021, well-nigh impossible. These are foreign organizations and individuals, operating abroad and often covertly, and they are often impecunious as well. So difficult is it to obtain monetary relief against covert foreign organizations like these that Congress has taken to passing legislation authorizing the payment of judgments against them from U.S. Treasury funds. E.g., Victims of Trafficking and Violence Protection Act of 2000, Pub.L. No. 106-386, § 2002, 114 Stat. 1464. But that can have no deterrent or incapacitative effect, whereas suits against financiers of terrorism can cut the terrorists’ lifeline.
And whether it makes good sense or not, the imposition of civil liability through the chain of incorporations is compelled by the statutory texts — as the panel determined in its first opinion. 291 F.3d at 1012-16. But in addition the panel placed a common law aiding and abetting gloss on section 2333. The panel was worried about a timing problem: section 2339A was not passed until 1994, and the defendants’ contributions to Hamas began earlier. But that is not a serious problem on the view we take of the standard for proving causation under section 2333; we shall see that the fact of contributing to a terrorist organization rather than the amount of the contribution is the keystone of liability.
Only because this is a very old case — David Boim was killed 12 years ago — does the 1994 effective date of section 2339A, two years before his killing, present an obstacle to liability, though only with respect to Salah and possibly the Holy Land Foundation (but we are vacating the judgment against the latter anyway, as we shall explain). For there is no doubt that the other defendants made contributions after section 2339A’s effective date. Salah, however, having been arrested by Israeli authorities in 1993 and not released until 1997, did not render material support to Hamas between the effective date of section 2339A and Boim’s killing, so the judgment against him must be reversed. New future cases will be affected by the timing issue, because few such cases will involve donations that were made after section 2333 was enacted in 1990 or re-enacted in 1992 but that ceased before 1994.
In addition to providing material support after the effective date of section 2339A, a donor to terrorism, to be liable under section 2333, must have known that the money would be used in preparation for or in carrying out the killing or attempted killing of, conspiring to kill, or inflicting bodily injury on, an American citizen abroad. We know that Hamas kills Israeli Jews; and Boim was an Israeli citizen, Jewish, living in Israel, and therefore a natural target for Hamas. But we must consider the knowledge that the donor to a terrorist organization must be shown to possess in order to be liable under section 2333 and the proof required to link the donor’s act to the injury sustained by the victim. The parties have discussed both issues mainly under the rubrics of “conspiracy” and “aiding and abetting.” Although those labels are significant primarily in criminal cases, they can be used to establish tort liability, see, e.g., Halberstam v. Welch, 705 F.2d 472 (D.C.Cir.1983); Restatement (Second) of Torts §§ 876(a), (b) (1979), and there is no impropriety in discussing them in reference to the liability of donors to terrorism under section 2333 just because that liability is primary. Primary liability in the form of material support to terrorism has the character of secondary liability. *692Through a chain of incorporations by reference, Congress has expressly imposed liability on a class of aiders and abettors.
When a federal tort statute does not create secondary liability, so that the only defendants are primary violators, the ordinary tort requirements relating to fault, state of mind, causation, and foreseeability must be satisfied for the plaintiff to obtain a judgment. See, e.g., Bridge v. Phoenix Bond & Indemnity Co., — U.S. -, 128 S.Ct. 2131, 2141-44, 170 L.Ed.2d 1012 (2008); Stoneridge Investment Partners, LLC v. Seientific-Atlanta, Inc., supra, 128 S.Ct. at 769; Holmes v. Securities Investor Protection Corp., 503 U.S. 258, 268-69, 112 S.Ct. 1311, 117 L.Ed.2d 532 (1992); Associated General Contractors of California, Inc. v. California State Council of Carpenters, 459 U.S. 519, 103 S.Ct. 897, 74 L.Ed.2d 723 (1983). But when the primary liability is that of someone who aids someone else, so that functionally the primary violator is an aider and abettor or other secondary actor, a different set of principles comes into play. Those principles are most fully developed in the criminal context, but we must be careful in borrowing from criminal law because the state-of-mind and causation requirements in criminal cases often differ from those in civil cases. For example, because the criminal law focuses on the dangerousness of a defendant’s conduct, the requirement of proving that a criminal act caused an injury is often attenuated and sometimes dispensed with altogether, as in the statutes that impose criminal liability on providers of material support to terrorism (18 U.S.C. §§ 2339A, B, and C), which do not require proof that the material support resulted in an actual terrorist act, or that punish an attempt (e.g., 18 U.S.C. § 1113) that the intended victim may not even have noticed, so that there is no injury. The law of attempt has no counterpart in tort law, United States v. Gladish, 536 F.3d 646, 648 (7th Cir.2008), because there is no tort without an injury. E.g., Rozenfeld v. Medical Protective Co., 73 F.3d 154, 155-56 (7th Cir.1996); Winskunas v. Bimbaum, 23 F.3d 1264, 1267 (7th Cir.1994).
So prudence counsels us not to halt our analysis with aiding and abetting but to go on and analyze the tort liability of providers of material support to terrorism under general principles of tort law. We begin by noting that knowledge and intent have lesser roles in tort law than in criminal law. A volitional act that causes an injury gives rise to tort liability for negligence if the injurer failed to exercise due care, period. But more is required in the case of intentional torts, and we can assume that since section 2333 provides for an automatic trebling of damages it would require proof of intentional misconduct even if the plaintiffs in this case did not have to satisfy the state-of-mind requirements of sections 2339A and 2332 (but they do).
Punitive damages are rarely if ever imposed unless the defendant is found to have engaged in deliberate wrongdoing. “Something more than the mere commission of a tort is always required for punitive damages. There must be circumstances of aggravation or outrage, such as spite or ‘malice,’ or a fraudulent or evil motive on the part of the defendant, or such a conscious and deliberate disregard of the interests of others that the conduct may be called wilful or wanton.” W. Page Keeton et al., Prosser and Keeton on the Law of Torts § 2, pp. 9-10 (5th ed.1984); see, e.g., Molzof v. United States, 502 U.S. 301, 305-07, 112 S.Ct. 711, 116 L.Ed.2d 731 (1992); Kemezy v. Peters, 79 F.3d 33, 35 (7th Cir.1996). Treble damages too, not being compensatory, tend to have a punitive aim. “The very idea of treble damages reveals an intent to punish past, and to deter future, unlawful conduct.” Texas *693Industries, Inc. v. Radcliff Materials, Inc., 451 U.S. 630, 639, 101 S.Ct. 2061, 68 L.Ed.2d 500 (1981); see also Vermont Agency of Natural Resources v. United States ex rel. Stevens, 529 U.S. 765, 784-86, 120 S.Ct. 1858, 146 L.Ed.2d 836 (2000); Zelinski v. Columbia 300, Inc., 335 F.3d 633, 642 (7th Cir.2003); Gorenstein Enterprises, Inc. v. Quality Care-USA, Inc., 874 F.2d 431, 435-36 (7th Cir.1989); United States v. Mackby, 261 F.3d 821, 830-31 (9th Cir.2001).
To give money to an organization that commits terrorist acts is not intentional misconduct unless one either knows that the organization engages in such acts or is deliberately indifferent to whether it does or not, meaning that one knows there is a substantial probability that the organization engages in terrorism but one does not care. “When the facts known to a person place him on notice of a risk, he cannot ignore the facts and plead ignorance of the risk.” Makor Issues & Rights, Ltd. v. Tellabs Inc., 513 F.3d 702, 704 (7th Cir.2008). That is recklessness and equivalent to recklessness is “wantonness,” which “has been defined as the conscious doing of some act or omission of some duty under knowledge of existing conditions and conscious that from the doing of such act or omission of such duty injury will likely or probably result.” Graves v. Wildsmith, 278 Ala. 228, 177 So.2d 448, 451 (1965); see also Landers v. School District No. 203, O’Fallon, 66 Ill.App.3d 78, 22 Ill.Dec. 837, 383 N.E.2d 645 (1978). “[I]n one case we read that ‘willful and wanton misconduct approaches the degree of moral blame attached to intentional harm, since the defendant deliberately inflicts a highly unreasonable risk of harm upon others in conscious disregard of it.’ Similarly, [another case] defines ‘willful and wanton’ as exhibiting ‘an utter indifference to or conscious disregard for’ safety.” Fagocki v. Algonquin/Lake-in-the-Hills Fire Protection District, 496 F.3d 623, 627 (7th Cir.2007) (citations omitted).
So it would not be enough to impose liability on a donor for violating section 2333, even if there were no state-of-mind requirements in sections 2339A and 2332, that the average person or a reasonable person would realize that the organization he was supporting was a terrorist organization, if the actual defendant did not realize it. That would just be negligence. But if you give a gun you know is loaded to a child, you know you are creating a substantial risk of injury and therefore your doing so is reckless and if the child shoots someone you will be liable to the victim. See Pratt v. Martineau, 69 Mass.App.Ct. 670, 870 N.E.2d 1122 (2007); Bowen v. Florida, 791 So.2d 44, 48-49 (Fla.App. 2001). That case should be distinguished from one in which the gun is given to an adult without adequately explaining the dangers — a case of negligent entrustment. To give a small child a loaded gun would be a case of criminal recklessness and therefore satisfy the state of mind requirement for liability under section 2333 and the statutes that it incorporates by reference. For the giver would know he was doing something extremely dangerous and without justification. “If the actor knows that the consequences are certain, or substantially certain, to result from his act, and still goes ahead, he is treated by the law as if he had in fact desired to produce the result.” Restatement, supra, § 8A, comment b. That you did not desire the child to shoot anyone would thus be irrelevant, not only in a tort case, see EEOC v. Illinois, 69 F.3d 167, 170 (7th Cir.1995), but in a criminal case. United States v. Fountain, 768 F.2d 790, 798 (7th Cir.1985); cf. United States v. Ortega, 44 F.3d 505, 508 (7th Cir.1995).
A knowing donor to Hamas — that is, a donor who knew the aims and activities of *694the organization — would know that Hamas was gunning for Israelis (unlike some other terrorist groups, Hamas’s terrorism is limited to the territory of Palestine, including Israel; see Council on Foreign Relations, “Hamas,” www.cfr.org/publication/ 8968/, visited Nov. 16, 2008), that Americans are frequent visitors to and sojourners in Israel, that many U.S. citizens live in Israel (American Citizens Abroad, an advocacy group for expatriates, reports on the basis of State Department data that in 1999 there were about 184,000 American citizens living in Israel, accounting for about 3.1 percent of the country’s population, www.aca.ch/amabroad.pdf, visited Nov. 16, 2008), and that donations to Ha-mas, by augmenting Hamas’s resources, would enable Hamas to kill or wound, or try to kill, or conspire to kill more people in Israel. And given such foreseeable consequences, such donations would “appear to be intended ... to intimidate or coerce a civilian population” or to “affect the conduct of a government by ... assassination,” as required by section 2331(1) in order to distinguish terrorist acts from other violent crimes, though it is not a state-of-mind requirement; it is a matter of external appearance rather than subjective intent, which is internal to the inten-der.
It is true that “the word ‘recklessness’ in law covers a spectrum of meaning, ranging from gross negligence in an accident case to the conduct of a robber in shooting at a pursuing policeman without aiming carefully.” Wright v. United States, 809 F.2d 425, 427 (7th Cir.1987). In tort law it sometimes connotes merely gross negligence and at other times requires only that the defendant have acted in the face of an unreasonable risk that he should have been aware of even if he wasn’t. But when, as in the passages we have quoted both from judicial opinions and from the Restatement, recklessness entails actual knowledge of the risk, the tort concept merges with the criminal concept, which likewise “generally permits a finding of recklessness only when a person disregards a risk of harm of which he is aware.” Farmer v. Brennan, 511 U.S. 825, 837, 114 S.Ct. 1970, 128 L.Ed.2d 811 (1994); see also Desnick v. American Broadcasting Cos., 233 F.3d 514, 517-518 (7th Cir.2000); American Law Institute, Model Penal Code § 2.02(2)(c) (1962) (defining recklessness as “consciously disregarding] a substantial and unjustifiable risk that the material element exists or will result from his conduct”).
Critically, the criminal (like the tort) concept of recklessness is more concerned with the nature and knowledge of the risk that the defendant creates than with its magnitude. The Court in Farmer v. Brennan spoke of an “excessive” risk, a “significant” risk, a “substantial” risk, and an “intolerable” risk, 511 U.S. at 837-38, 842-43, 846, 114 S.Ct. 1970, the Model Penal Code of a “substantial and unjustifiable” risk, and the Restatement of an “unreasonable” risk, Restatement, supra, § 500, rather than assigning a minimum probability to the risk. These are relative terms; what is excessive, intolerable, etc., depends on the nature of the defendant’s conduct. Ordinarily, it is true, the risk is great in a probabilistic sense; for the greater it is, the more likely it is to materialize and so give rise to a lawsuit or a prosecution and thus be mentioned in a judicial opinion. The greater the risk, moreover, the more obvious it will be to the risk taker, enabling the trier of fact to infer the risk taker’s knowledge of the risk with greater confidence, see, e.g., Farmer v. Brennan, supra, 511 U.S. at 842, 114 S.Ct. 1970; Duckworth v. Franzen, 780 F.2d 645, 652 (7th Cir.1985), though, as the Farmer decision emphasizes, subject to rebuttal. 511 U.S. at 837-42, 114 S.Ct. 1970.
*695But probability isn’t everything. The risk that one of the workers on a project to build a bridge or a skyscraper will be killed may be greater than the risk that a driver will be killed by someone who flings rocks from an overpass at the cars traveling on the highway beneath. But only the second risk, though smaller, is deemed excessive and therefore reckless. McNabb v. State, 887 So.2d 929, 974-75 (Ala.Crim. App.2001). (The first risk might not even be negligent.) As we explained in United States v. Boyd, 475 F.3d 875, 877 (7th Cir.2007) (emphasis added), “firing multiple shots from a powerful gun ... in the downtown of a large city at a time when pedestrians ... are known to be in the vicinity creates a risk of harm that, while not large in 'probabilistic terms, is ‘substantial’ relative to the gratuitousness of the defendant’s actions.... An activity is reckless when the potential harm that it creates ... is wildly disproportionate to any benefits that the activity might be expected to confer.... The emotional gratification that defendant Boyd derived from shooting into the night, though perhaps great, is not the kind of benefit that has weight in the scales when on the other side is danger to life and limb, even if the danger is limited, as it was here.” Lennon v. Metropolitan Life Ins. Co., 504 F.3d 617, 623 (6th Cir.2007), says that the risk must be “weighed against the lack of social utility of the activity” in adjudging its reasonableness. See also Orban v. Vaughn, 123 F.3d 727, 733 (3d Cir.1997).
So if you give a person rocks who has told you he would like to kill drivers by dropping them on cars from an overpass, and he succeeds against the odds in killing someone by this means, you are guilty of providing material support to a murderer, or equivalently of aiding and abetting — for remember that when the primary violator of a statute is someone who provides assistance to another he is functionally an aider and abettor. The mental element required to fix liability on a donor to Hamas is therefore present if the donor knows the character of that organization.
The Court also said in Farmer v. Brennan that it was no defense that “he [a particular prison official] did not know that the complainant was especially likely to be assaulted by the specific prisoner who eventually committed the assault.” 511 U.S. at 843, 114 S.Ct. 1970. That brings us to our next question — the standard of causation in a suit under section 2333.
It is “black letter” law that tort liability requires proof of causation. But like much legal shorthand, the black letter is inaccurate if treated as exceptionless. We made that point explicitly, with the aid of an example, in Maxwell v. KPMG LLP, 520 F.3d 713, 716 (7th Cir.2008): “when two fires join and destroy the plaintiffs property and each one would have destroyed it by itself and so was not a necessary condition ... each of the firemakers (if negligent) is [nevertheless] liable to the plaintiff for having ‘caused’ the injury. Kingston v. Chicago & N.W. Ry., 191 Wis. 610, 211 N.W. 913 (Wis.1927)” (emphasis added); see also United States v. Feliciano, 45 F.3d 1070, 1075 (7th Cir.1995). (A “necessary condition” is another term for a “but for” cause. Maxwell v. KPMG LLP, supra, 520 F.3d at 716.)
The multiple-fire example and the principle that subtends it were explained at greater length in United States v. Johnson, 380 F.3d 1013, 1016 (7th Cir.2004): “[T]wo defendants each start a fire, and the fires join and destroy the plaintiffs house; either fire, however, would have destroyed his house. Each defendant could therefore argue that he should not be hable for the damage because it would have occurred even if he had not set his fire; but the law rejects the argument.... *696[I]n the famous old ease of Cook v. Minneapolis, St. Paul & Sault Ste. Mane Ry., 98 Wis. 624, 74 N.W. 561, 564 (1898), we read that ‘it is no defense for a person against whom negligence which causes damages is established, to prove that without fault on his part the same damage would have resulted from the negligent act of the other, but each is responsible for the entire damage.’ See also Anderson v. Minneapolis, St. Paul & Sault Ste. Marie Ry., 146 Minn. 430, 179 N.W. 45, 49 (1920); Collins v. American Optometric Ass’n, 693 F.2d 636, 640 n. 4 (7th Cir.1982); Housing 21, L.L.C. v. Atlantic Home Builders Co., 289 F.3d 1050, 1056-57 (8th Cir.2002); Sanders v. American Body Armor & Equipment, Inc., 652 So.2d 883, 884-85 (Fla.App.1995); Garrett v. Grant School Dist. No. 124, 139 Ill.App.3d 569, 93 Ill.Dec. 874, 487 N.E.2d 699, 706 (1985); Hart v. Browne, 103 Cal.App.3d 947, 163 Cal.Rptr. 356, 363-64 (1980); W. Page Keeton et al., Prosser and Keeton on the Law of Torts § 41, pp. 266-67 (5th ed.1984). The tort-feasor cannot avoid liability by pointing to an alternative unlawful cause of the damage that he inflicted.... [S]ince neither fire was a sine qua non of the plaintiffs injury, it could be argued that neither fire maker had committed a tort. Tort law rejects this conclusion for the practical reason that tortious activity that produces harm would go unsanctioned otherwise.” The Prosser treatise also recognizes the multiple-fire case as one in which the plaintiff is not required to prove “but for” causation. Keeton et al., supra, § 41, pp. 266-68; cf. Edward J. Schwartzbauer and Sidney Shindell, “Cancer and The Adjudicative Process: The Interface of Environmental Protection and Toxic Tort Law,” 14 Am. J.L. & Med. 1, 31-32 (1988).
In the fire cases the acts of each defendant are sufficient conditions of the resulting injury, though they are not necessary conditions (that is, they are not but-for causes). But in Summers v. Tice, 33 Cal.2d 80, 199 P.2d 1 (1948), where two hunters negligently shot their rifles at the same time and a third hunter was hit by one of the bullets, it could not be determined which hunter’s gun the bullet had come from and so it could not be proved by a preponderance of the evidence that either of the shooters was the injurer in either a sufficient-condition or a necessary-condition sense; for each hunter, the probability that he had caused the injury was only 50 percent, since one of the shots had missed. Nevertheless both defendants were held jointly and severally liable to the injured person. See Restatement, supra, § 433B(3) and comment f; Smith v. Cutter Biological, 72 Haw. 416, 823 P.2d 717, 725 (1991); In re “Agent Orange” Product Liability Litigation, 597 F.Supp. 740, 822-23 (E.D.N.Y.1984).
Similarly, if several firms spill toxic waste that finds its way into groundwater and causes damage to property but it is impossible to determine which firm’s spill caused the damage, all are liable. See, e.g., Chem-Nuclear Systems, Inc. v. Bush, 292 F.3d 254, 259-60 (D.C.Cir.2002); United States v. Alcan Aluminum Corp., 964 F.2d 252, 267-69 (3d Cir.1992); Michie v. Great Lakes Steel Division, 495 F.2d 213, 217-18 (6th Cir.1974); Landers v. East Texas Salt Water Disposal Co., 151 Tex. 251, 248 S.W.2d 731, 734 (1952); Phillips Petroleum Co. v. Hardee, 189 F.2d 205, 211-12 (5th Cir.1951); 2 Frank P. Grad, Treatise on Environmental Law § 3.02 (2007); Kenneth S. Abraham, “The Relation Between Civil Liability and Environmental Regulation: An Analytical Overview,” 41 Washburn L.J. 379, 386-87 (2002). Even if the amount of pollution caused by each party would be too slight to warrant a finding that any one of them had created a nuisance (the common law basis for treating pollution as a tort), “pol*697lution of a stream to even a slight extent becomes unreasonable [and therefore a nuisance] when similar pollution by others makes the condition of the stream approach the danger point. The single act itself becomes wrongful because it is done in the context of what others are doing.” Keeton et al., supra, § 52, p. 354.
In all these cases the requirement of proving causation is relaxed because otherwise there would be a wrong and an injury but no remedy because the court would be unable to determine which wrongdoer inflicted the injury. If “each [defendant] bears a like relationship to the event” and “each seeks to escape liability for a reason that, if recognized, would likewise protect each other defendant in the group, thus leaving the plaintiff without a remedy,” the attempt at escape fails; each is liable. Id., § 41, p. 268.
But we must consider the situation in which there is uncertainty about the causal connection between the wrongful conduct of all potential tortfeasors and the injury. Suppose in our first case that there was a third fire, of natural origin (the result of a lightning strike, perhaps), and it alone might have sufficed to destroy the plaintiffs house. One might think the law would require the plaintiff to prove that it was more likely than not that had it not been for the defendants’ negligence, his house would not have burned down — the Are of natural origin would have petered out before reaching it. Instead the law requires proof only that there was a substantial probability that the defendants’ fires (or rather either of them) were the cause. See, e.g., Anderson v. Minneapolis, St. Paul & Sault Ste. Marie Ry., supra, 179 N.W. at 46; Restatement, supra, § 432(2) (“if two forces are actively operating, one because of the actor’s negligence, the other not because of any misconduct on his part, and each of itself is sufficient to bring about harm to another, the actor’s negligence may be found to be a substantial factor in bringing it about”); see also id., illustration 3.
Our final example is Keel v. Hainline, 331 P.2d 397 (Okla.1958). Thirty to forty junior high school students showed up one day for their music class, but the instructor failed to show so the kids began throwing wooden erasers, chalk, and even a Coke bottle at each other. One of the students was struck in the eye by an eraser, and sued. One of the defendants, Keel, apparently had not thrown anything. But he had retrieved some of the erasers after they had been thrown and had handed them back to the throwers. There was no indication that Keel had handed the eraser to the kid who threw it at the plaintiff and injured her, but the court deemed that immaterial. It was enough that Keel had participated in the wrongful activity as a whole. He thus was liable even though there was no proven, or even likely, causal connection between anything he did and the injury. “ ‘One who commands, directs, advises, encourages, procures, instigates, promotes, controls, aids, or abets a wrongful act by another has been regarded as being as responsible as the one who commits the act so as to impose liability upon the former to the same extent as if he had performed the act himself.’ ” Id. at 401. The court did not use the term “material support,” but in handing erasers to the throwers Keel was providing them with material support in a literal sense. It was enough to make him liable that he had helped to create a danger; it was immaterial that the effect of his help could not be determined — that his acts could not be found to be either a necessary or a sufficient condition of the injury.
The cases that we have discussed do not involve monetary contributions to a wrongdoer. But then criminals and other *698intentional tortfeasors do not usually solicit voluntary contributions. Terrorist organizations do. But this is just to say that terrorism is sui generis. So consider an organization solely involved in committing terrorist acts and a hundred people all of whom know the character of the organization and each of whom contributes $1,000 to it, for a total of $100,000. The organization has additional resources from other, unknown contributors of $200,000 and it uses its total resources of $300,000 to recruit, train, equip, and deploy terrorists who commit a variety of terrorist acts one of which kills an American citizen. His estate brings a suit under section 2333 against one of the knowing contributors of $1,000. The tort principles that we have reviewed would make the defendant jointly and severally liable with all those other contributors. The fact that the death could not be traced to any of the contributors (as in the example the Supreme Court gave in Farmer v. Brennan) and that some of them may have been ignorant of the mission of the organization (and therefore not liable under a statute requiring proof of intentional or reckless misconduct) would be irrelevant. The knowing contributors as a whole would have significantly enhanced the risk of terrorist acts and thus the probability that the plaintiffs decedent would be a victim, and this would be true even if Hamas had incurred a cost of more than $1,000 to kill the American, so that no defendant’s contribution was a sufficient condition of his death.
This case is only a little more difficult because Hamas is (and was at the time of David Boim’s death) engaged not only in terrorism but also in providing health, educational, and other social welfare services. The defendants other than Salah directed their support exclusively to those services. But if you give money to an organization that you know to be engaged in terrorism, the fact that you earmark it for the organization’s nonterrorist activities does not get you off the liability hook, as we noted in a related context in Hussain v. Mukasey, 518 F.3d 534, 538-39 (7th Cir.2008); see also Singh-Kaur v. Ashcroft, 385 F.3d 293, 301 (3d Cir.2004). The reasons are twofold. The first is the fungibility of money. If Hamas budgets $2 million for terrorism and $2 million for social services and receives a donation of $100,000 for those services, there is nothing to prevent its using that money for them while at the same time taking $100,000 out of its social services “account” and depositing it in its terrorism “account.” Kilbum v. Socialist People’s Libyan Arab Jamahiriya, 376 F.3d 1123, 1130 (D.C.Cir.2004).
Second, Hamas’s social welfare activities reinforce its terrorist activities both directly by providing economic assistance to the families of killed, wounded, and captured Hamas fighters and making it more costly for them to defect (they would lose the material benefits that Hamas provides them), and indirectly by enhancing Ha-mas’s popularity among the Palestinian population and providing funds for indoctrinating schoolchildren. See, e.g., Justin Magouirk, “The Nefarious Helping Hand: Anti-Corruption Campaigns, Social Service Provision, and Terrorism,” 20 Terrorism & Political Violence 356 (2008); Eli Berman & David D. Laitin, “Religion, Terrorism, and Public Goods: Testing the Club Model” 7-10 (National Bureau of Econ. Research Working Paper No. 13725, 2008). Anyone who knowingly contributes to the nonviolent wing of an organization that he knows to engage in terrorism is knowingly contributing to the organization’s terrorist activities. And that is the only knowledge that can reasonably be required as a premise for liability. To require proof that the donor intended that his contribution be used for terrorism — to make a benign intent a defense — would as *699a practical matter eliminate donor liability except in cases in which the donor was foolish enough to admit his true intent. It would also create a First Amendment Catch-22, as the only basis for inferring intent would in the usual case be a defendant’s public declarations of support for the use of violence to achieve political ends.
Although liability under section 2333 is broad, to maintain perspective we note two cases that fall on the other side of the liability line. One is the easy case of a donation to an Islamic charity by an individual who does not know (and is not reckless, in the sense of strongly suspecting the truth but not caring about it) that the charity gives money to Hamas or some other terrorist organization.
The other case is that of medical (or other innocent) assistance by nongovernmental organizations such as the Red Cross and Doctors Without Borders that provide such assistance without regard to the circumstances giving rise to the need for it. Suppose an Israeli retaliatory strike at Hamas causes so many casualties that the local medical services cannot treat all of them, and Doctors Without Borders offers to assist. And suppose that many of the casualties that the doctors treat are Hamas fighters, so that Doctors Without Borders might know in advance that it would be providing medical assistance to terrorists.
However, section 2339A(b)(l) excludes “medicine” from the definition of “material resources.” And even if the word should be limited (an issue on which we take no position) to drugs and other medicines, an organization like Doctors Without Borders would not be in violation of section 2333. It would be helping not a terrorist group but individual patients, and, consistent with the Hippocratic Oath, with no questions asked about the patients’ moral virtue. It would be like a doctor who treats a person with a gunshot wound whom he knows to be a criminal. If doctors refused to treat criminals, there would be less crime. But the doctor is not himself a criminal unless, besides treating the criminal, he conceals him from the police (like Dr. Samuel Mudd, sentenced to prison for trying to help John Wilkes Booth, Lincoln’s assassin, elude capture) or violates a law requiring doctors to report wounded criminals. The same thing would be true if a hospital unaffiliated with Hamas but located in Gaza City solicited donations.
Nor would the rendering of medical assistance by the Red Cross or Doctors Without Borders to individual terrorists “appear to be intended ... to intimidate or coerce a civilian population” or “affect the conduct of a government by ... assassination,” and without such appearance there is no international terrorist act within the meaning of section 2331(1) and hence no violation of section 2333. Nor is this point limited to the rendering of medical assistance. For example, UNRWA (the United Nations Relief and Works Agency for Palestine Refugees in the Near East) renders aid to Palestinian refugees that is not limited to medical assistance to individual refugees, www.un.org/unrwa/english.html (visited Nov. 16, 2008). But so far as one can glean from its website (see id. and www.un.org/unrwa/allegations/index.html, also visited Nov. 16, 2008), it does not give money to organizations, which might be affiliates of Hamas or other terrorist groups; it claims to be very careful not to employ members of Hamas or otherwise render any direct or indirect aid to it. Id.
To the objection that the logic of our analysis would allow the imposition of liability on someone who with the requisite state of mind contributed to a terrorist organization in 1995 that killed an American abroad in 2045, we respond first that *700that is not this case — the interval here was at most two years (1994, when section 2339A was enacted, to 1996, when Boim was killed) — and second that the imposition of liability in the hypothetical case would not be as outlandish, given the character of terrorism, as one might think. (There would of course be no defense of statute of limitations, since the limitations period would not begin to run until the tort was committed, and that would not occur until the injury on which suit was based was inflicted.) Terrorism campaigns often last for many decades. Think of Ireland, Sri Lanka, the Philippines, Colombia, Kashmir — and Palestine, where Arab terrorism has been more or less continuous since 1920. Seed money for terrorism can sprout acts of violence long after the investment. In any event, whether considerations of temporal remoteness might at some point cut off liability is not an issue we need try to resolve in this case.
An issue to which the first panel opinion gave much attention (see 291 F.3d at 1021-27), but which received little attention from the parties afterward, is brought into focus by our analysis of the elements of a section 2333 violation. That is whether the First Amendment insulates financiers of terrorism from liability if they do not intend to further the illegal goals of an organization like Hamas that engages in political advocacy as well as in violence. If the financier knew that the organization to which it was giving money engaged in terrorism, penalizing him would not violate the First Amendment. Otherwise someone who during World War II gave money to the government of Nazi Germany solely in order to support its anti-smoking campaign could not have been punished for supporting a foreign enemy.
But it is true that “an organization is not a terrorist organization just because one of its members commits an act of armed violence without direct or indirect authorization, even if his objective was to advance the organization’s goals, though the organization might be held liable to the victim of his violent act.” Hussain v. Mukasey, supra, 518 F.3d at 538. That is the principle of NAACP v. Claiborne Hardware Co., 458 U.S. 886, 920, 102 S.Ct. 3409, 73 L.Ed.2d 1215 (1982). The defendants in the present case could not be held liable for acts of violence by members of Hamas that were not authorized by Hamas. Nor would persons be liable who gave moral rather than material support, short of incitement, to violent organizations that have political aims. As intimated earlier in this opinion, a person who gives a speech in praise of Hamas for firing rockets at Israel is exercising his freedom of speech, protected by the First Amendment. See, e.g., Communist Party of Indiana v. Whitcomb, 414 U.S. 441, 447-49, 94 S.Ct. 656, 38 L.Ed.2d 635 (1974); Brandenburg v. Ohio, 395 U.S. 444, 447-48, 89 S.Ct. 1827, 23 L.Ed.2d 430 (1969) (per curiam). But as Hamas engages in violence as a declared goal of the organization, anyone who provides material support to it, knowing the organization’s character, is punishable (provided he is enchained by the chain of statutory incorporations necessary to impose liability under section 2333) whether or not he approves of violence.
Enough about the liability standard. We have now to consider its application to the facts. That turns out to be straightforward, except with respect to one of the defendants, the Holy Land Foundation, about which we can be brief because of the thoroughness of the panel’s consideration. See 511 F.3d at 720-33. A principal basis for the district court’s finding that the Foundation had violated the statute was the court’s giving collateral estop-pel effect to findings made in Holy Land Foundation for Relief & Development v. *701Ashcroft, 219 F.Supp.2d 57 (D.D.C.2002), affirmed, 333 F.3d 156 (D.C.Cir.2003). The panel was unanimous that this ruling was erroneous.
In 2001 the Secretary of the Treasury determined that the Foundation “acts for or on behalf of’ Hamas, and an order freezing the Foundation’s funds was issued. The Foundation sued in the District of Columbia. The district court there found that the Secretary’s finding was not “arbitrary and capricious” (the standard of review) and upheld the blocking order. Although the court recited extensive evidence that the Foundation knew that Ha-mas was and had long been a terrorist organization, 219 F.Supp.2d at 69-75, and it appears that most or perhaps all of the evidence related to its knowledge before 1996 when David Boim was killed, the validity of the blocking order did not depend on the Foundation’s knowledge. 511 F.3d at 731; see Executive Order 13244, 66 Fed.Reg. 49079 (Sept. 23, 2001); Garry W. Jenkins, “Soft Power, Strategic Security, and International Philanthropy,” 85 N. Car. L.Rev. 773, 808-09 (2007); Jennifer Lynn Bell, “Terrorist Abuse of Non-Profits and Charities: A Proactive Approach to Preventing Terrorist Financing,” 17 Kan. J.L. & Public Policy 450, 458-59 (2008). If someone is giving money to an organization that the government knows to be a terrorist organization, any subsequent gift can be blocked whether or not the donor knows (or agrees with the government concerning) the nature of the recipient.
Even if the decision of the district court in the District of Columbia were read as finding that the Foundation knew that Ha-mas was a terrorist organization (and, as the court also found, that the Holy Land Foundation made contributions to Hamas after the effective date of 18 U.S.C. § 2339A, 219 F.Supp.2d at 70-71), such a finding would not have been essential to the judgment upholding the blocking order — and essentiality is at the heart of collateral estoppel. Arizona v. California, 530 U.S. 392, 414, 120 S.Ct. 2304, 147 L.Ed.2d 374 (2000); Montana v. United States, 440 U.S. 147, 159, 99 S.Ct. 970, 59 L.Ed.2d 210 (1979); H-D Michigan, Inc. v. Top Quality Service, Inc., 496 F.3d 755, 760 (7th Cir.2007); Central Hudson Gas & Electric Corp. v. Empresa Naviera Santa S.A., 56 F.3d 359, 368 (2d Cir.1995); Restatement (Second) of Judgments § 27 (1982). If a finding is unnecessary to the judgment, the appellant has no reason to challenge it and if he does the appellate court has no reason to review it because it is irrelevant to the appeal — and so the appellant would not have his (full) day in court.
So the judgment against the Foundation must be reversed and the case against it remanded for further proceedings to determine its liability. The judgment against Salah must also be reversed, as we explained earlier. Regarding the remaining defendants, the American Muslim Society and the Quranic Literacy Institute, the judgment of the district court was in our view correct. The activities of the American Muslim Society are discussed at length in the district court’s second opinion. See 340 F.Supp.2d at 906-13. There we learn that while its activities included donating money to the Holy Land Foundation, there was much else besides. Moreover, the fact that the Foundation may not have known that Hamas was a terrorist organization (implausible as that is) would not exonerate the American Muslim Society, which did know and in giving money to the Foundation was deliberately tunneling money to Hamas. The funnel doesn’t have to know what it’s doing to be an effective funnel.
Nor should donors to terrorism be able to escape liability because terrorists and *702their supporters launder donations through a chain of intermediate organizations. Donor A gives to innocent-appearing organization B which gives to innocent-appearing organization C which gives to Hamas. As long as A either knows or is reckless in failing to discover that donations to B end up with Hamas, A is liable. Equally important, however, if this knowledge requirement is not satisfied, the donor is not liable. And as the temporal chain lengthens, the likelihood that a donor has or should know of the donee’s connection to terrorism shrinks. But to set the knowledge and causal requirement higher than we have done in this opinion would be to invite money laundering, the proliferation of affiliated organizations, and two-track terrorism (killing plus welfare). Donor liability would be eviscerated, and the statute would be a dead letter.
With regard to the Quranic Literacy Institute, the district court, after denying the Institute’s motion for summary judgment, 340 F.Supp.2d at 929, submitted the case against the Institute to a jury trial but instructed the jury that Hamas was responsible for the murder of David Boim. The jury was left to decide whether the Institute had knowingly provided material support to Hamas. The jury found the Institute liable. By deciding not to participate in the trial, the Institute waived any objection it might have had to the jury instructions or the jury’s findings.
In any event, the only factual determination underlying the judgment against the Institute, as against the American Muslim Society, that might be questioned — and was by the panel — was the determination, made by the district court on summary judgment, that Hamas had been responsible for the murder. The panel thought that the district judge had considered inadmissible evidence that the two terrorists who shot Boim were in fact members of Hamas.
Here is the panel’s critique of the principal though not only evidence of their membership:
To show that the murder of David Boim was the work of Hamas, the Boims submitted the declaration of Dr. Ruven [sic ] Paz, a former member of the Israeli security community who describes himself as an expert in terrorism and counter-terrorism, Islamic movements in the Arab and Islamic world, Palestinian Islamic groups, and Palestinian society and politics. Based on his review of various exhibits submitted in connection with this case, his independent research, and his knowledge of how Hamas and other Islamic terror organizations operate, Paz concluded that Hinawi and Al-Sharif had murdered David Boim, that Hinawi and Al-Sharif were members of Hamas at the time they killed Boim, and that Hamas itself had accepted responsibility for the murder....
In concluding that Al-Sharif was a member of Hamas and that Hamas had taken responsibility for the murder, Paz relied heavily on information set forth on certain websites that he attributed to Hamas. Paz explained that Hamas publicly acknowledges its terrorist acts and identifies its “martyrs” as a way to promote itself and to recruit new members. According to Paz, internet websites are a means by which Hamas disseminates such information. Paz’s declaration asserts that scholars, journalists, and law enforcement routinely rely on the website postings of terrorist organizations for what they reveal about the activities of those organizations. Looking to certain websites whose content he asserts is controlled by Hamas, Paz found statements indicating that Hamas had taken responsibility for the Beit-El attack that *703took David Boim’s life and that Al-Sharif was one of the participants in this attack. Paz repeated these statements in his declaration.
Paz’s reliance upon, and his recounting of, internet website postings demand a certain caution in evaluating his prospective testimony. Such postings would not be admissible into evidence for their truth absent proper authentication, and this would typically require some type of proof that the postings were actually made by the individual or organization to which they are being attributed — in this case, Hamas — as opposed to others with access to the website. Paz’s declaration identifies the websites from which he quotes as ones controlled by Hamas, but it does not describe the basis for his conclusion, and consequently his declaration does not permit any independent assessment of the purported links between these sites and Hamas and the source of the postings that he recounts. Of course, the rules of evidence do not limit what type of information an expert may rely upon in reaching his opinion; even if that information would not otherwise be admissible in a court proceeding, an expert witness may rely upon it so long as it is the type of information on which others in the field reasonably rely. Indeed, Rule 703 now expressly permits the expert to disclose such information to the jury, provided the court is satisfied that its helpfulness in evaluating the expert’s opinion substantially outweighs its prejudicial effect. Nonetheless, a judge must take care that the expert is not being used as a vehicle for circumventing the rule against hearsay. Where, as here, the expert appears to be relying to a great extent on web postings to establish a particular fact, and where as a result the factfinder would be unable to evaluate the soundness of his conclusion without hearing the evidence he relied on, we believe the expert must lay out, in greater detail than Paz did, the basis for his conclusion that these websites are in fact controlled by Hamas and that the postings he cites can reasonably and reliably be attributed to Hamas.
Paz’s conclusion that Hinawi was responsible for the murder of David Boim was based in significant part on two documents related to Hinawi’s trial and sentencing by a Palestinian Authority tribunal: (1) a set of notes prepared by a U.S. foreign service officer who attended Hinawi’s trial in February 1998, and (2) an Arabic-language document purporting to be the written verdict reflecting Hinawi’s conviction and sentence. The foreign service officer’s notes indicate that Hinawi was tried in open proceedings for participating in a terrorist act and acting as an accomplice in the killing of David Boim, that he was afforded counsel by the tribunal, that he contended in his defense that his friend AI-Sharif was the gunman and that Al-Sharif exploited his friendship with Hi-nawi by asking him to drive the car, and that he was convicted on both charges and sentenced to ten years. Paz’s declaration accepts these documents as genuine and relies principally on them for the proposition that Hinawi participated in David Boim’s murder and was convicted by the Palestinian Authority tribunal for the same.
Once again we have concerns about whether the record as it stands lays an appropriate foundation for these documents. We can assume that the report of a U.S. government official who, in the course of his duties, observed a trial in a foreign tribunal may constitute proof of what occurred in that proceeding. We also have no doubt that a properly authenticated, official report of a judgment *704issued by a foreign tribunal constitutes adequate proof of that judgment. The difficulty we have with Paz’s reliance upon these documents is that they have not been properly authenticated. The foreign service officer’s notes are unsigned and reveal nothing about the circumstances under which they were prepared. The document that we are told is the official verdict is entirely in Arabic, is not readily evident as an official document, and is unaccompanied by an English translation. There is a single cover note, on the letterhead of the U.S. Consulate General in Jerusalem, which accompanies these documents and explains what they are. But the cover note itself is unsigned and does not even identify its author. This is unacceptable. We assume that Paz knows more about these documents and that he would not have relied upon them if he had doubts about their authenticity. But given that Paz relies almost exclusively on these documents as proof of Hinawi’s complicity in Boim’s murder, and because a factfinder could not evaluate the soundness of Paz’s conclusion without knowing what these documents say, an appropriate foundation must be laid for these documents before the conclusions that Paz has drawn from these documents may be admitted.
511 F.3d at 752-54 (citations omitted).
We accept the panel majority’s description of the infirmities of the evidence on which Reuven Paz (formerly research director of Shin Bet, Israel’s domestic security agency) based his expert opinion. But we do not agree that the district court abused its discretion in allowing the opinion into evidence. As the quoted passage acknowledges (albeit grudgingly, in its warning against using an expert witness “as a vehicle for circumventing the rule against hearsay”), an expert is not limited to relying on admissible evidence in forming his opinion. Fed.R.Evid. 703; Wendler & Ezra, P.C. v. American Int’l Group, Inc., 521 F.3d 790, 791 (7th Cir.2008); In re James Wilson Associates, 965 F.2d 160, 172-73 (7th Cir.1992); United States v. Locascio, 6 F.3d 924, 938 (2d Cir.1993). That would be a crippling limitation because experts don’t characteristically base their expert judgments on legally admissible evidence; the rules of evidence are not intended for the guidance of experts. Biologists do not study animal behavior by placing animals under oath, and students of terrorism do not arrive at their assessments solely or even primarily by studying the records of judicial proceedings. Notice, moreover, that there was no need for the plaintiffs to prove that both Al-Sherif and Hinawi were complicit in Boim’s death; if either was complicit and a member of Hamas, that is enough to fix responsibility on Hamas for killing Boim.
In dissenting from the panel’s ruling Judge Evans offered an assessment of Paz’s evidence (see 511 F.3d at 758) that we find persuasive. An expert on terrorism in the Arab world, fluent in Arabic, Paz explained that the websites of Islamic movements and Islamic terrorist organizations have long been accepted by security experts as valid, important, and indeed indispensable sources of information. Terrorist organizations rely on the web to deliver their messages to their adherents and the general public. The United States Institute for Peace, a nonpartisan federal institution created by Congress, published an extensive report, submitted to the district court along with Paz’s declaration, on the use of the Internet by terrorists. And — critically—the defendants presented no evidence to contradict Paz: no evidence that the killing of Boim was not a Hamas hit. Had they thought Paz had mistranslated the Arabic judgment against Hinawi, they could have provided the district court *705with their own translation. Had they doubted that Paz can identify a Hamas website (he gave the web addresses of several of them), they could have presented testimony to that effect. Paz’s 12-page declaration is detailed, concrete, and backed up by a host of exhibits. The district court did not abuse its discretion in admitting his evidence; and with it in the record and nothing on the other side the court had no choice but to enter summary judgment for the plaintiffs with respect to Hamas’s responsibility for the Boim killing.
To summarize, the judgment of the district court is affirmed except with respect to (1) Salah, as to whom the judgment is reversed with instructions to enter judgment in his favor; (2) the Holy Land Foundation, as to which the judgment is reversed and the case remanded for further proceedings consistent with this opinion; and (3) the award of attorneys’ fees— for we adopt the panel’s criticisms of that award, 511 F.3d at 749-50, and anyway the award will have to be adjusted because of the further proceedings on remand that we are ordering.
AFFIRMED IN PART, REVERSED IN PART, AND Remanded.