AMENDED OPINION
At the request of the majority of the en banc court and in order to avoid the need for an en banc rehearing, the original panel amends its opinion of August 4, 2008, by deleting its discussion of counsel’s failure to seek an independent mental health expert and the failure of counsel to object to the Presentence Report. Therefore, the sole basis for the issuance of the writ of habeas corpus is counsel’s failure to investigate mitigating factors.
*525 This is an appeal in an Ohio death penalty case by the prisoner, Van Hook, seeking habeas corpus relief under 28 U.S.C. § 2254. On the evening of February 18, 1985, petitioner Robert Van Hook arrived at a bar frequented by male homosexuals in Cincinnati, Ohio. He left the bar with David Self, and the two proceeded to Selfs apartment. Once there, Van Hook strangled Self to the point of unconsciousness. He then brutally killed Self, stabbing him several times in the head and abdomen. After stealing a few items from Selfs apartment, Van Hook fled to Florida, where he was apprehended over a month later and subsequently confessed to the murder. 1
Back in Ohio, Van Hook waived his rights to a trial by jury, and he pleaded not guilty and not guilty by reason of insanity. The three-judge panel, elected under Ohio law, rejected this defense and found him guilty of aggravated murder and aggravated robbery, which made him eligible for the death penalty under Ohio Rev.Code Ann. § 2929.04(A) (West 2008). Finding that the mitigating evidence did not outweigh the aggravators, the three-judge panel imposed the death penalty instead of life imprisonment.
Though he did not deny killing Self, Van Hook asserted unsuccessfully a variety of errors both on direct appeal and in state post-conviction proceedings.
See State v. Van Hook,
The district court denied the petition on all asserted claims of error. Our panel then reversed the judgment of the district court because under
Edwards v. Arizona,
After a careful review of the record, we conclude that Van Hook’s trial counsel was ineffective during the mitigation phase of the trial, thereby violating his rights under the Sixth Amendment, as interpreted by the Supreme Court in three cases,
Strickland v. Washington,
*526
The Antiterrorism and Effective Death Penalty Act of 1996 (“AEDPA”), 28 U.S.C. § 2244,
et seq.,
was signed into law and became effective on April 24, 1996. Because Van Hook filed his habeas corpus petition on October 10, 1995, the Act does not apply as a constraint on our interpretation and application of constitutional standards in this death penalty case.
See Mopes v. Coyle,
Since 1984, the standard for whether counsel’s ineffectiveness fell below the minimum requirements of the Sixth Amendment contains two components: (1) the deficient performance of counsel and (2) the resulting prejudice to the defendant.
Strickland v. Washington,
After
Strickland,
this Court and the Supreme Court made clear in a number of cases that counsel in death cases should follow closely the ABA standards referred to above.
See Wiggins v. Smith,
*527
Van Hook is correct that the performance of his trial counsel was deficient during the mitigation phase because his attorneys failed to fully investigate and present evidence of all the potential mitigating factors that could have reduced his sentence from death to life imprisonment. Counsel has a duty to investigate fully, unless counsel makes a reasonable strategic choice to limit the investigation.
See Strickland,
Our Court’s precedents make clear that a partial but ultimately incomplete mitigation investigation is inadequate.
See Dickerson v. Bagley,
Both this Court and the Supreme Court have also held counsel’s performance deficient when counsel’s last-minute investigation resulted in overlooking potentially powerful mitigating evidence.
See Williams v. Taylor,
Applying these clear rules to the performance of Van Hook’s trial counsel, it is clear that counsel’s investigation into and presentation of mitigating evidence was deficient. While Van Hook’s trial attorneys uncovered a little information about his traumatic childhood experience in their last-minute investigation, many of the most important details were not discovered and therefore were never presented to the sen-tencer. Significantly, trial counsel’s investigation failed to reveal that Van Hook’s parents repeatedly beat him (J.A. at 1619), that he had witnessed his father attempt to kill his mother several times (J.A. at 1619), and that his mother was committed to a psychiatric hospital when he was between four and five years old (J.A. at 1570). These details of his childhood are even more unsettling and potentially mitigating than the omitted family background evidence in Dickerson, where the omitted evidence simply included the fact that the defendant had been taunted at school and referred to as the “moron.” The details about Van Hook’s childhood, which were uncovered later by the more thorough investigation of Van Hook’s habeas counsel, demonstrate that trial counsel’s investigation into Van Hook’s background was never finished because the investigation was conducted at the last minute.
Van Hook’s counsel’s mitigation investigation did not “begin quickly” before trial. Rather, after the guilt phase, counsel started a last minute investigation for the mitigation hearing. (J.A. at 4400-04.) His attorneys thus spent far less time preparing than the counsel in Williams, where counsel was deemed ineffective for not having begun preparing for mitigation until a week before the guilt phase of trial. This cursory preparation for mitigation also parallels the preparations by counsel in Glenn, which this Court held to be objectively unreasonable.
By not performing the sort of extensive, thorough investigation that is a minimum requirement of trial counsel in these cases, the performance of Van Hook’s counsel turned up very little of the available mitigation evidence. Contrary to the perception of the state appellate court, this omitted evidence was much more than “merely cumulative.”
State v. Van Hook,
Nor can counsel’s decision to terminate the mitigation investigation before uncovering this information be considered a reasonable, strategic decision. Considering the information that they had already learned about Van Hook’s abusive family background, counsel certainly had reason to suspect that much worse details existed. But his attorneys decided not to interview or even contact Van Hook’s step-sister, his paternal uncle, two of his paternal aunts, his maternal uncle, and the psychiatrist who treated his mother when she was committed. All of these individuals could have helped his counsel narrate the true story of Van Hook’s childhood experiences in mitigation. All of them would have been willing to testify on his behalf. (J.A. 1569-73,1619-26.) 2
*529
Failing to complete a mitigation investigation when additional family witnesses are available is not sound trial strategy; neither is waiting until four days before the mitigation hearing to begin the investigation.
See Williams v. Taylor,
For an ineffective assistance of counsel claim to succeed, counsel must not only have performed deficiently, but that performance must have prejudiced the defendant.
See Strickland v. Washington,
Counsel’s deficient performance prevented the three-judge panel from learning fully about the two statutory mitigating factors that were the strongest in his case — his traumatic family background and his mental illness.
Had his attorneys performed a complete mitigation investigation, that panel would have learned how Van Hook was often beaten by his parents, how he saw his father try to kill his mother, and how his mother was committed to a psychiatric hospital when he was a young child. (J.A. at 1570, 1619.) Further, had his attorneys sought out the available family members willing to help tell his story, that panel would have heard additional “first-hand accounts from those who knew [Van Hook] best.”
Powell v. Collins,
Our conclusion is bolstered by the fact that Ohio is a so-called “weighing” state, which means that the aggravating circum
*530
stances must outweigh the mitigating factors in order to impose the death penalty. Ohio Rev.Code Ann. § 2929.04(B). Van Hook’s conviction only qualified for one of Ohio’s statutory aggravating circumstances: his offense “was committed while [he] was committing ... aggravated robbery.”
Id.
§ 2929.04(A)(7). Thus, the introduction of more available mitigating evidence could certainly have tipped the scales in favor of his life. The threshold for finding prejudice in this case is thus lower than in previous cases, where we found prejudice despite the trial courts’ having found multiple aggravating circumstances. Cf
. Dickerson v. Bagley,
We believe that “there is a reasonable probability that, absent the errors, the sentencer ... would have concluded that the balance of aggravating and mitigating circumstances did not warrant death.”
Strickland,
Because we have decided that counsel failed to conduct a full mitigation investigation and present available mitigating evidence to the sentencer and thereby offered constitutionally ineffective assistance to Van Hook at the sentencing phase of the trial, we will not decide, and we therefore pretermit, the remaining issues. For the foregoing reasons, we reverse the decision of the district court and remand the case to the district court with instructions to issue a writ of habeas corpus vacating Van Hook’s death sentence unless the State conducts a new penalty phase proceeding within 180 days of remand.
Notes
. For a more detailed recitation of the facts surrounding the murder and Van Hook's apprehension, see
State v. Van Hook,
. Counsel's decision not to introduce additional family background witnesses also can
*529
not be justified under the strategy of attempting to prevent the sentencer from learning about prior criminal convictions. In several of our recent cases where few mitigation witnesses were introduced, we refused to find counsel’s performance deficient in large part because these witnesses likely would have had to reveal the defendant's history of violence.
See Durr v. Mitchell,
