concurring:
I concur in Judge Tashima’s opinion. I write separately to note that, if we were not reversing on the grounds stated in the majority opinion, we would necessarily have to reverse and remand with instructions to the district court to hold an evi-dentiary hearing on Garceau’s claim of ineffective assistance of counsel during the sentencing phase. Indeed, the warden conceded as much during oral argument of this case.
We review the district court’s decision to deny an evidentiary hearing for an abuse of discretion. Lawson v. Borg, 60 F.3d 608, 611 (9th Cir.1995). In this case, the district court erred in denying Garceau’s request for an evidentiary hearing concerning his claim of ineffective assistance of counsel at the penalty phase.
“To obtain an evidentiary hearing on an ineffective assistance of counsel claim, a habeas petitioner must establish that (1) his allegations, if proven, would constitute a colorable claim, thereby entitling him to relief and (2) the state court trier of fact has not, after a full and fair hearing, reliably found the relevant facts.” Correll v. Stewart, 137 F.3d 1404, 1411 (9th Cir. 1998). “In addition, ... if the petitioner has failed to develop material facts in state court proceedings, he or she must demonstrate [1] adequate cause for his or her failure and [2] actual prejudice resulting from that failure.” Id.
Garceau did not develop the facts supporting his ineffective assistance of counsel claims in state court. However, he has shown both cause and prejudice justifying his failure.
A proper request for an evidentiary hearing in state court satisfies the “cause” requirement.
When a state court denies an evidentia-ry hearing on a colorable ineffective assistance of counsel claim after proper request, a habeas petitioner has fulfilled the [Keeney v.] Tamayo-Reyes [504 U.S. 1, 112 S.Ct. 1715, 118 L.Ed.2d 318 (1992)] “cause” requirement. Simply put, the state cannot successfully oppose a petitioner’s request for a state court evidentiary hearing, then argue in federal habeas proceedings that the petitioner should be faulted for not succeeding.
Correll, 137 F.3d at 1413. Therefore, Gar-ceau has satisfied the “cause” requirement by requesting a state evidentiary hearing on these issues. See id.
Because the Tamayo-Reyes prejudice prong is coextensive with the ineffective assistance of counsel prejudice prong under Strickland v. Washington, 466 U.S. 668, 104 S.Ct. 2052, 80 L.Ed.2d 674 (1984), Garceau will have satisfied the prejudice prong under Tamayo-Reyes if he establishes a colorable ineffective assistance of counsel claim. See Correll, 137 F.3d at 1414.
The parties do not dispute that Garceau did not receive a state evidentiary hearing on these issues; therefore, a state court has not “reliably found the relevant facts,” see id. at 1411, and he will be entitled to an evidentiary hearing if he establishes a col-orable claim.
“To establish a colorable claim of ineffective assistance of counsel, [petitioner] must demonstrate [1] that his counsel’s *779performance at trial was deficient, and [2] that the deficient performance prejudiced his defense.” Id. (citing Strickland, 466 U.S. at 687, 104 S.Ct. 2052).
In this case, Garceau’s counsel performed deficiently in preparing for and conducting Garceau’s sentencing hearing. Counsel failed to investigate mitigating evidence relating to drug addiction and post-traumatic stress disorder (“PTSD”). Counsel also failed to investigate and rebut aggravating evidence regarding Garceau’s participation in an alleged kidnapping.
“[WJhere counsel is on notice that his client may be mentally impaired, counsel’s failure to investigate his client’s mental condition as a mitigating factor in a penalty phase hearing, without a supporting strategic reason, constitutes deficient performance.” Hendricks v. Calderon, 70 F.3d 1082, 1043 (9th Cir.1995).
Counsel testified that he knew of Gar-ceau’s cocaine use and that he “was aware that cocaine use caused significant alternations in mental functioning and behavior, including that cocaine was widely recognized to cause psychosis as a result of chronic use.” Nonetheless, he did not investigate the extent of Garceau’s use, nor did he question the witnesses about Gar-ceau’s use or behavioral changes although even the prosecution’s witnesses described Garceau’s paranoid and delusional behavior. Furthermore, he hired an expert who could not document or diagnose the impact of chronic cocaine dependence and was not a psychopharmacologist. No strategic reason supported counsel’s failure to investigate Garceau’s drug addiction. See id.
Similarly, counsel failed to investigate or present evidence of PTSD. Although counsel stated that he was not aware that Garceau suffered from PTSD, he did know that Garceau served in Vietnam and abused heavy amounts of cocaine. Dr. Craig W. Haney, a psychology professor and an attorney, declared that at the time of the arrest and trial “it was common knowledge among virtually all mental health professionals and most capital attorneys with whom [he] was familiar that such experiences [as combat service in Vietnam] could produce longlasting psychological problems of the sort [Garceau] was experiencing and, also, that many persons suffering from these psychological problems used alcohol and illicit drugs in an effort to ‘self-medicate’ and blunt the pain and mask the emotional and behavioral consequences of the post-traumatic stress they were experiencing.” Given the combat service, the drug use, the violence of the attacks, and the statements by his drug partners about paranoia and delusional behavior, counsel should have known to investigate PTSD as a possible mitigating factor. The failure to do so falls “outside the wide range of professionally competent assistance.” Strickland, 466 U.S. at 690,104 S.Ct. 2052.
The decision to not present mitigating evidence at a capital sentencing hearing “should be the product of a reasoned choice. Counsel has ‘a duty to make reasonable investigations or to make a reasonable decision that makes particular investigations unnecessary.’” Cornil, 137 F.3d at 1412 (quoting Strickland, 466 U.S. at 691, 104 S.Ct. 2052). In this case, counsel did not make a reasoned choice; he simply failed to investigate potentially mitigating evidence. See id.
Additionally, counsel performed “outside the wide range of professionally competent assistance,” Strickland, 466 U.S. at 690, 104 S.Ct. 2052, in utterly failing to investigate and rebut the aggravating evidence of Garceau’s involvement in an alleged kidnapping. The prosecution presented considerable evidence during the sentencing hearing about this event. Garceau was initially charged as the driver in a kidnap*780ping; however, the charges were dropped after the defense attorney produced a witness who claimed that the alleged victim got into the truck voluntarily. Counsel in this case did not contact the prior attorney, did not interview any of the prosecution’s witnesses, and did not attempt to find other percipient witnesses. In fact, one of the witnesses, the co-defendant in the kidnapping, even contacted counsel, but counsel refused to speak with him and just told him to arrive when he had been subpoenaed. During the sentencing hearing, counsel did not even bother to present evidence that the charges had been dismissed. Counsel’s utter failure to investigate this aggravating evidence constitutes constitutionally deficient performance. See Correll, 137 F.3d at 1412-13.
Counsel’s deficient performance also prejudiced Garceau. “To establish the prejudice prong of the Strickland standard, ‘[t]he defendant must show that there is a reasonable probability that, but for counsel’s unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome.’” Hendricks, 70 F.3d at 1036 (alteration in original) (quoting Strickland, 466 U.S. at 694, 104 S.Ct. 2052). Failing to reasonably investigate potentially mitigating circumstances without a reasonable tactical explanation constitutes prejudice. Correll, 137 F.3d at 1413 (“[A]bsent any reasonable investigation into potentially mitigating circumstances and any reasonable tactical explanation, we must conclude [petitioner] has made a colorable claim of ineffective assistance of counsel at his capital sentencing.”).
Counsel put on no expert testimony at all during the penalty phase.1 The jury learned of Garceau’s drug use through the prosecution witnesses, but counsel failed to put Garceau’s drug use, social history, and Vietnam experience before the jury in a way that could explain his addiction and his behavior. In failing to do so, his performance prejudiced Garceau. See Caro v. Calderon, 165 F.3d 1223, 1227-28 (9th Cir. 1999); see also Wallace v. Stewart, 184 F.3d 1112, 1116 (9th Cir.1999) (“Does an attorney have a professional responsibility to investigate and bring to the attention of mental health experts who are examining his client, facts that the experts do not request? The answer, at least at the sentencing phase of a capital case, is yes.”), cert. denied, 528 U.S. 1105, 120 S.Ct. 844, 145 L.Ed.2d 713 (2000).
Furthermore, the prosecution focused intently on the alleged kidnapping during the sentencing hearing. In closing, the prosecutor argued a trend of escalating violence that culminated in the three murders: “consider ... whether this defendant is a man who has committed isolated acts of criminal conduct or whether this offender has rather developed a pattern of criminal conduct that began with the burglary and ultimately exploded into what I’m about to tell you is three murders.” State Ct. Rep. Tr. of Sentencing Hr’g at 172. The kidnapping was an essential “first outward manifestation of violence toward another person” in this argument. State Ct. Rep. Tr. of Sentencing Hr’g at 173. Because of the kidnapping’s pivotal role in the prosecution’s argument, counsel’s utter failure to investigate and rebut the prosecution’s characterization of the incident as a crime of violence, at least by mentioning that the charges had been dismissed because a witness claimed that the alleged victim voluntarily accompanied the *781defendants, is “sufficient to undermine confidence in the outcome.” Strickland, 466 U.S. at 694, 104 S.Ct. 2052.
Garceau has established a colorable claim of ineffective assistance of counsel during his sentencing hearing. He has also demonstrated cause and prejudice, see Tamayo-Reyes, 504 U.S. at 6, 112 S.Ct. 1715, for his failure to develop related facts in state court. Therefore, the district court erred in denying his request for an evidentiary hearing on these issues. See Cornil, 137 F.3d at 1412.
. In fact, counsel relied exclusively on the testimony of two family members and the cross-examination of the prosecution’s witnesses — most of whom testified about the alleged kidnapping.