OPINION OF THE COURT
SCIRICA, Circuit Judge.Mitchell Robertson entered a conditional guilty plea1 to possession of ammunition by a convicted felon, a violation of 18 U.S.C. § 922(g)(1).2 The sole issue on appeal is whether police officers had reasonable suspicion to stop a public bus on which Robertson was traveling and search for and seize a handgun and ammunition. We will affirm.
I.
On April 29, 1999, at 12:10 p.m., Philadelphia Police Captain Joseph Sullivan of the 35th Police District and Officer Joseph Carolyn, his driver, responded to a radio call that police officers were pursuing two male robbery suspects on the run, in the area of 18th Street and 66th Avenue. At least one of the suspects was allegedly armed. A second radio report described the men as African-American, one wearing a white shirt and reddish pants, the other a white or gray shirt and dark pants, possibly blue jeans. After traveling ten to twelve city blocks, Captain Sullivan and Officer Carolyn met up with several other officers in the 6600 block of Gratz Street, one-half block west of 18th Street. Captain Sullivan and Officer Carolyn left their unmarked patrol car to speak with the other officers.
At this point Captain Sullivan saw two men running “in the eastbound actual traffic lanes” of 66th Avenue, crossing the intersection of Gratz Street.3 Believing these two men fit the descriptions he had just received, Captain Sullivan instructed Officer Carolyn to return to their car, pick him up, and follow the suspects. The patrol car was parked a quarter of the way up Gratz Street, facing north, and Officer Carolyn had to back up to get onto 66th Avenue. A line of buildings and houses *166caused Captain Sullivan to lose sight of the two men after they turned the corner onto 66th Avenue. When Officer Carolyn arrived with the car, approximately one minute later, they proceeded east on 66th Avenue.
Just then, a van pulled alongside Captain Sullivan’s window, and the van driver gestured to roll down the police car’s window. Captain Sullivan described the van driver as a heavy-set African-American male in his late forties to early fifties. The van driver informed Captain Sullivan the two men he was “looking for” had boarded a SEPTA bus4 a few blocks away on 66th Avenue: “Officer, them two guys you’re looking for just got on that bus.” Captain Sullivan did not ask the van driver’s name, address, or telephone number. The two officers chased after the eastbound SEPTA bus, stopped and boarded it. Two to three minutes elapsed between the time Officer Carolyn obtained the car and the time the officers boarded the bus.
Among the twelve to fifteen passengers on the bus, Captain Sullivan saw two men matching the descriptions of the robbery suspects. Both were seated in the rear of the bus. Captain Sullivan testified that Robertson, one of the two men, was wearing a grayish shirt with dark pants5 and the person sitting next to Robertson was wearing a white shirt and red pants.6
Upon boarding the SEPTA bus, Captain Sullivan made eye contact with Robertson. He saw Robertson remove an item from his waistband with his right hand, reach over the passenger seated next to him, and place the item behind the seat in front of him and to his right — on top of the bus’s wheel well. Based on his experience, Captain Sullivan believed Robertson was trying to hide a concealed weapon. Captain Sullivan drew his weapon and ordered Robertson to lie on the floor. A search of the wheel well by another officer revealed a loaded five-shot break-open revolver.
Other officers brought the robbery victim to the scene, but the victim was unable to identify Robertson and his companion as the robbers. The recovered weapon was of indeterminate age and therefore, could not form the basis of a weapons possession charge. See 18 U.S.C. § 921(a)(3) (1994).7 Robertson was charged with illegal possession of ammunition by a convicted felon, a violation of 18 U.S.C. § 922(g)(1).8 After a hearing on Robertson’s motion to suppress, the District Court held the police officers “were justified in stopping the SEPTA bus, having formed the ... reasonable belief that the two robbery suspects had gotten on *167the bus and believing that at least one of them had a gun.” This appeal followed.
II.
The District Court had jurisdiction under 18 U.S.C. § 3231. We have jurisdiction under 28 U.S.C. § 1291.
III.
The central question on appeal is whether Captain Sullivan had reasonable suspicion to stop and board the SEPTA bus on which Robertson was traveling. As in all difficult suppression cases, we must consider the totality of the circumstances, including the police officer’s knowledge, experience, and common sense judgments about human behavior. The Fourth Amendment prevents “unreasonable searches and seizures.” U.S. Const, amend. IV. Generally, for a seizure to be reasonable under the Fourth Amendment, it must be effectuated with a warrant based on probable cause. Katz v. United States, 389 U.S. 347, 356-57, 88 S.Ct. 507, 19 L.Ed.2d 576 (1967). But under the “narrowly drawn authority” of Terry v. Ohio, 392 U.S. 1, 27, 88 S.Ct. 1868, 20 L.Ed.2d 889 (1968), an officer without a warrant “may, consistent with the Fourth Amendment, conduct a brief, investigatory stop when the officer has a reasonable, articulable suspicion that criminal activity is afoot.” Wardlow, 528 U.S. at 123, 120 S.Ct. 673.
To determine whether reasonable suspicion exists, we must consider the “ ‘totality of the circumstances — the whole picture.’ ” United States v. Sokolow, 490 U.S. 1, 8, 109 S.Ct. 1581, 104 L.Ed.2d 1 (1989) (quoting United States v. Cortez, 449 U.S. 411, 417, 101 S.Ct. 690, 66 L.Ed.2d 621 (1981)); see also United States v. Arvizu, 534 U.S. 266, 122 S.Ct. 744, 750-51, 151 L.Ed.2d 740 (2002) (“This process allows officers to draw on their own experience and specialized training to make inferences from and deductions about the cumulative information available to them that might well elude an untrained person.” (quotation and citation omitted)). In United States v. Nelson, 284 F.3d 472 (3d Cir.2002), we described Arvizu as follows: “In the Supreme Court’s most recent pronouncement on the Fourth Amendment reasonable suspicion standard, it accorded great deference to the officer’s knowledge of the nature and the nuances of the type of criminal activity that he had observed in his experience, almost to the point of permitting it to be the focal point of the analysis.” Id. at 482.
Moreover, we are appropriately reluctant to “second-guess” investigative decisions made by officers in hot pursuit of criminal suspects. E.g., United States v. Valentine, 232 F.3d 350, 355 (3d Cir.2000) (“The officers knew the suspect was still in the vicinity, and had they stalled for more lengthy questioning of the informant, the armed suspect could have escaped detection.”), cert. denied, 532 U.S. 1014, 121 S.Ct. 1748, 149 L.Ed.2d 670 (2001); United States v. Brown, 159 F.3d 147, 149 (3d Cir.1998) (“A police officer may conduct a warrantless stop and frisk if specific and articulable facts, together with all rational inferences, suggest that the suspect was involved in criminal activity.”) (summarizing Terry). The Supreme Court has held “the determination of reasonable suspicion must be made on common sense judgments and inferences about human behavior.” Wardlow, 528 U.S. at 125, 120 S.Ct. 673.
IV.
The District Court found the stop of the SEPTA bus was supported by reasonable suspicion and the search and seizure was valid. We exercise plenary re*168view over the District Court’s conclusions regarding reasonable suspicion. We review the District Court’s factual findings for clear error. United States v. Riddick, 156 F.3d 505, 509 (3d Cir.1998). We exercise plenary review over the District Court’s application of law to the facts of this case. Id.
As noted, calculating whether an officer has reasonable suspicion to warrant a stop and search is often an imprecise judgment. Arvizu, 122 S.Ct. at 751 (“Our cases have recognized that the concept of reasonable suspicion is somewhat abstract.”); Wardlow, 528 U.S. at 125, 120 S.Ct. 673 (“[W]e cannot reasonably demand scientific certainty from judges or law enforcement officers where none exists.”). The determination is often made under exigent circumstances requiring quick, decisive reactions. Under this set of facts, Captain Sullivan had a reasonable suspicion that the two suspects he viewed sprinting through the streets of Philadelphia, in close proximity to the scene of the armed robbery, had committed the crime. See Alabama v. White, 496 U.S. 325, 330, 110 S.Ct. 2412, 110 L.Ed.2d 301 (1990) (“Reasonable suspicion is a less demanding standard than-probable cause not only in the sense that reasonable suspicion can be established with information that is different in quantity or content than that required to establish probable cause, but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause.”).
The “total picture” in this case demonstrates Captain Sullivan reasonably suspected the men who boarded the bus were the suspected armed robbers. In reaching this determination, Captain Sullivan relied on his experience and training, indispensable to his evaluation of reasonable suspicion. Cf. Terry, 392 U.S. at 30, 88 S.Ct. 1868; see also Ornelas v. United States, 517 U.S. 690, 699, 116 S.Ct. 1657, 134 L.Ed.2d 911 (1996) (“[A] police officer views the facts through the lens of his police experience and expertise.”).
It is well settled that reasonable suspicion can be based on information gathered from another person. Adams v. Williams, 407 U.S. 143, 147, 92 S.Ct. 1921, 32 L.Ed.2d 612 (1972). Robertson contends Captain Sullivan improperly relied on statements from the van driver before deciding to stop the SEPTA bus. We disagree. Information provided by the van driver was reliable because it was provided by a witness “reporting what he had observed moments ago, not what he learned from stale or second-hand sources.” Valentine, 232 F.3d at 354. The van driver spoke directly to Captain Sullivan, who had at least a fleeting “opportunity to assess[his] credibility and demeanor.” Id. at 350. The van driver was in the area where an armed robbery had just occurred. He was driving on the same street where two “armed” fleeing men had been spotted. Cf. Brown, 159 F.3d at 150 (suspect’s presence in “close proximity to the crime scene a few minutes after” a crime supported a finding of reasonable suspicion). He initiated contact with Captain Sullivan and, without being prompted, said the “two guys” pursued had just boarded the SEPTA bus. “[I]n light of the total circumstances,” the information was “sufficiently trustworthy.” Valentine, 232 F.3d at 355.
No doubt in perfect hindsight and with more time, Captain Sullivan might well have asked the bystander more questions. But elaboration or corroboration in these circumstances can delay — and even terminate — effective pursuit. Valentine, 232 F.3d at 355. This concern is not abstract. At the suppression hearing Captain Sullivan was asked by defense counsel whether *169he had seen the van driver before, whether the van driver was an “officer,” and whether he took the van driver’s name, address or telephone number. To all these inquiries, Captain Sullivan answered, “No, I was a little more concerned with catching that bus.” If Captain Sullivan had waited to determine if the van driver had a basis for his statement, the fleeing suspects may well have escaped. It is legally insignificant that Robertson was not ultimately identified by the victim as the armed robber. Florida v. J.L., 529 U.S. 266, 271, 120 S.Ct. 1375, 146 L.Ed.2d 254 (2000) (“The reasonableness of official suspicion must be measured by what the officers knew before they conducted their search.”).
Nor do we believe Captain Sullivan’s “hot pursuit” of the suspects had turned “cold” by the time he boarded the bus. Only two to three minutes passed between the time Officer Carolyn began backing down Gratz Street and the time Captain Sullivan stopped the bus. Under this set of facts, the “hot” pursuit remained quite warm. Nor is there any allegation or hint that Captain Sullivan’s “hot pursuit” of Robertson was a pretext. Captain Sullivan reasonably thought he was chasing armed robbers through Philadelphia city streets. His decision to stop the bus was based on a “common sense judgment.” Wardloiu, 528 U.S. at 125, 120 S.Ct. 673.
Robertson characterizes the van driver as an “anonymous informant.” We disagree. We view this as essentially a hot pursuit case, aided by a bystander’s informative tip. For Fourth Amendment purposes, the information the van driver provided Captain Sullivan, coupled with the other circumstances, justified a “brief investigatory stop” of the bus. Accord Valentine, 232 F.3d at 353. Regardless, the information conveyed by the van driver contained several indicia of reliability, distinguishing it from the truly anonymous tip in Florida v. J.L. See 529 U.S. at 271, 120 S.Ct. 1375 (police cannot rely on “the bare report of an unknown, unaccountable informant who has neither explained how he knew [certain information] nor supplied any basis for believing he had inside information about [the suspect]”).
We cannot'divorce the information provided by the van driver from the surrounding circumstances of the hot pursuit. The Supreme Court has rejected this type of bifurcated analysis:
The [Court of Appeals for the' Ninth Circuit] appeared to believe that each observation by [a border patrol agent] that was by itself readily susceptible to an innocent explanation was entitled to “no' weight.” Terry, however, precludes this sort of divide-and-conquer analysis. The officer in Terry observed the petitioner and his companions repeatedly walk back and forth, look into a store ’ window, and Confer with one another. Although each of the series of acts was “perhaps innocent in itself,” we held that, taken together, they “warranted further investigation.”
Arvizu, 122 S.Ct. at 751 (citations omitted); see also Nelson, 284 F.3d at 484 (reiterating that the circumstances surrounding an anonymous tip — “its content and import, and the knowledge of the officer receiving it”- — must be considered to determine whether reasonable suspicion existed).
Furthermore, we believe United States v. Roberson, 90 F.3d 75 (3d Cir.1996), is inapposite. In Roberson, the Philadelphia Police Department 911 officer received an “anonymous tip” of criminal activity at the 2100 block of Chelten Avenue. Id', at 76. Police officers, who were unaware of the activity before receiving the call, approached a person matching the tipster’s profile. Id. We reversed the district *170court’s refusal to suppress evidence obtained through a subsequent search, largely because the police, “dealing with an anonymous and bare-bones tip,” had “no basis for assessing either the reliability of the informant or the grounds on which the informant believed that a crime was being committed.” Id. at 80.
Roberson is distinguishable in at least three significant ways. First, the investigation in Roberson was initiated because of an anonymous telephone “tip.” Here, Captain Sullivan was already pursuing persons matching a description, provided by other police officers, of two men who had just committed an armed robbery and were sprinting through a specific area. Having observed the fleeing suspects board the SEPTA bus, the bystander told Captain Sullivan where the men he “was looking for” had gone. Second, in Roberson, “[t]he officers could have set up surveillance of the defendant” in order to corroborate the “non-predictive, anonymous tip they received.” 90 F.3d at 81. Here, Captain Sullivan, in hot pursuit, did not have time to ask for details without risking the suspects’ disappearance. Finally, in Roberson we expressly declined to extend our holding beyond the facts presented, namely, when an anonymous tip suggests a defendant is selling drugs. 90 F.3d at 81 n. 4 (“We do not address whether such a tip is sufficient to create reasonable suspicion when the tip involves an allegation that the defendant was carrying a gun rather than dealing drugs. Under those circumstances, a different rule may apply.”).
In sum, we believe the totality of the circumstances demonstrates Captain Sullivan had reasonable suspicion to stop the SEPTA bus on which Robertson was riding. On these facts, Captain Sullivan reasonably relied on the credible information the van driver provided and made a “common sense” judgment consistent with constitutional requirements.
V.
Next we consider whether the search for and seizure of Robertson’s firearm and ammunition was consistent with the Fourth Amendment. As noted, Captain Sullivan had the requisite reasonable suspicion to stop the public bus on which Robertson was riding. Given the possibility that the suspects might have been carrying weapons,9 Captain Sullivan testified that when boarding the bus, he was concerned for his personal safety. As the Supreme Court held, “there must be a narrowly drawn authority to permit a reasonable search for weapons for the protection of the police officer, where he has reason to believe that he is dealing with an armed and dangerous individual, regardless of whether he has probable cause to arrest the individual for a crime.” Terry, 392 U.S. at 27, 88 S.Ct. 1868. A limited search of the suspects for weapons was justified under these circumstances.
But more significantly, Captain Sullivan saw Robertson remove an item from his waistband, reach over the adjacent passenger, and place the item on top of the bus’s wheel well. Captain Sullivan reasonably believed Robertson was trying to hide a concealed weapon. Therefore, Robertson also posed a threat to the bus passengers’ safety. Cf. Terry, 392 U.S. at 27, 88 S.Ct. 1868:
The officer need not be absolutely certain that the individual is armed; the issue is whether a reasonably prudent *171man in the circumstances would be warranted in the belief that his safety or that of others was in danger. And in determining whether the officer acted reasonably in such circumstances, due weight must be given, not to his inchoate and unparticularized suspicion, or “hunch,” but to the specific reasonable inferences which he is entitled to draw from the facts in light of his experience.
See also Nelson, 284 F.3d at 483 (noting the “Supreme Court’s consistent prior teaching that an officer, in determining whether there is reasonable suspicion, may take into account reports of an active threat, including the presence and use of dangerous weapons”). Given Robertson’s movements on the bus, which were observed by Captain Sullivan, the search for and ultimate seizure of Robertson’s handgun and ammunition were justified.
VI.
We reserve the broader question whether police need reasonable suspicion to stop a public bus.10 Accord United States v. Leon, 468 U.S. 897, 905, 104 S.Ct. 3405, 82 L.Ed.2d 677 (1984) (“Although it is undoubtedly within our power to consider the question ... that question has not been briefed or argued; and it is also within our authority, which we choose to exercise, to take the case as it comes to us.... ”).11
The Supreme Court, in a case involving an allegedly unconstitutional search on a public bus, recently reiterated that “for the most part, per se rules are inappropriate in the Fourth Amendment context.” United States v. Drayton, — U.S. -, 122 S.Ct. 2105, 2111, 153 L.Ed.2d 242 (2002) (holding the proper inquiry considers the totality of the circumstances). In Drayton, the Supreme Court rejected a categorical rule that evidence obtained during suspicionless drug interdiction searches on public buses must be suppressed unless police officers advise passengers of their rights not to cooperate and to refuse consent. Id. at 2111-12; see also id. at 2112 (“It is beyond question that had this encounter occurred on the street, it would be constitutional. The fact that an encounter takes place on a bus does not on its own transform standard police questioning of citizens into an illegal seizure.”). The proper test is the totality of the circumstances. As noted, once on the bus Captain Sullivan reasonably believed he saw Robertson remove a firearm from his waistband and place it on top of the bus’s wheel well. Under these circumstances, the search and seizure here is not unlawful under the Fourth Amendment.
VII.
For the foregoing reasons, we will affirm the judgment of the District Court.
. Under Fed.R.Crim.P. 11(a)(2), "a defendant may enter a conditional plea of guilty or nolo contendere, reserving in writing the right, on appeal from the judgment, to review of the adverse determination of any specified pretrial motion.” Robertson entered a conditional guilty plea under Rule 11, preserving his right to appeal the denial of his motion to suppress. See also United States v. Zudick, 523 F.2d 848, 851 (3d Cir.1975).
. He was sentenced to sixty-six months' imprisonment, three years of supervised release, a $500 fine, and a $100 special assessment.
.The two men Captain Sullivan saw were apparently running toward the robbery scene. But the direction of their flight may or may not be legally consequential. Fleeing robbers may have changed direction for a certain purpose. See, e.g., Illinois v. Wardlow, 528 U.S. 119, 124, 120 S.Ct. 673, 145 L.Ed.2d 570 (2000) ("Headlong flight — wherever it occurs — is the consummate act of evasion; it is not necessarily indicative of wrongdoing, but it is certainly suggestive of such.”). The direction the suspects were running remains one of many factors to consider in evaluating the "totality of the circumstances.” Here, we do not believe it negates the reasonable suspicion that these men were the alleged robbers.
. The Southeastern Pennsylvania Transportation Authority provides public bus transportation for the City of Philadelphia.
. The color of Robertson's clothing that day remains somewhat in dispute. Two other officers testified Robertson was wearing blue or dark jeans and a gray or dirty white shirt with black lettering on the front. Photographs taken at police headquarters show Robertson wearing a black T-shirt and jacket. Robertson’s prison receipt lists a black T-shirt, black jeans, and a multi-colored jacket.
. There is no dispute this person matched the description of the second suspect.
. Federal firearms statutes do not regulate the possession of firearms manufactured prior to 1898. The manufacture date of Robertson's firearm could not be established, so no federal prosecution for the gun could ensue. But the government proceeded with the ammunition count, which has no chronological limitation. See 18 U.S.C. §§ 921(a)(16), 921(a)(17).
. Robertson, also known as “Mitchell Robinson” and "Bryheer McMichael,” possessed five live rounds of ammunition, loaded in a Spanish break-open revolver with an obliterated serial number. Robertson has twice been convicted of crimes punishable by more than a j'ear in prison.
. To reiterate, in the radio report Captain Sullivan heard, officers broadcast that they were in pursuit of two males in connection with a robbery, and that at least one of the males was armed with a handgun.
. Even assuming Robertson had an expectation of privacy on this public bus, the firearm was located on the bus’s wheel well, not, for example, in luggage Robertson carried. To that extent, the search was of the bus itself, not of Robertson's person or belongings.
. Compare Delaware v. Prouse, 440 U.S. 648, 653, 99 S.Ct. 1391, 59 L.Ed.2d 660 (1979) (“stopping an automobile and detaining its occupants constitute a ‘seizure’ [for the Fourth Amendment]”), and United States v. Hernandez-Zuniga, 215 F.3d 483, 486-87 (5th Cir.2000) (assuming that a stop of a bus by the police constituted a seizure) (citing cases), with Florida v. Bostick, 501 U.S. 429, 436, 111 S.Ct. 2382, 115 L.Ed.2d 389 (1991) (holding that if police board a bus during a regular stop, the relevant inquiry is “whether a reasonable person would feel free to decline the officer's requests or otherwise terminate the encounter”), and United States v. Gonzales, 979 F.2d 711, 712-13 (9th Cir.1992) (boarding of bus implicated no constitutional rights where it occurred at a stoplight and transpired “pursuant to an agreement between the bus company and the border patrol”).