The Supreme Court overturned the sentence of a death row inmate in Pennsylvania yesterday, ruling that his attorneys failed to investigate readily available evidence of child abuse and mental illness that might have persuaded the jury to sentence him to life imprisonment.

The 5 to 4 decision showed that the court continues to actively supervise the performance of defense counsel in capital cases. The majority acknowledged that attorneys for Ronald Rompilla had made some effort to delve into their client's troubled past during his 1988 murder trial -- but faulted them for failing to dig even deeper.

The court noted that prosecutors twice told the lawyers that they planned to tell the jury about Rompilla's prior rape conviction, yet not until after the second warning did the defense team give even a partial look at the file from that case. The file contained documentation of the beatings and neglect Rompilla suffered as a child, as well as possible schizophrenia. Instead, the lawyers relied only on statements by Rompilla himself, his family and three mental health professionals.

"It flouts prudence to deny that a defense lawyer should try to look at a file he knows the prosecution will cull for aggravating evidence, let alone when the file is sitting in the trial courthouse, open for the asking," Justice David H. Souter wrote in the opinion for the court.

No reasonable lawyer would have made such a decision, Souter wrote, and as a result Rompilla's constitutional right to effective assistance of counsel was violated. Pennsylvania must now either give him a new sentencing hearing or agree to sentence him to life in prison, Souter wrote.

The ruling was in keeping with concerns about inadequate capital defense counsel expressed by members of the court in recent years, including public statements by two of the justices who joined Souter in yesterday's majority, Sandra Day O'Connor and Ruth Bader Ginsburg. Justices John Paul Stevens and Stephen G. Breyer also joined the majority.

It was the second time in three years that the court found a constitutional violation in the failure of defense lawyers to delve into the troubled past of a client; in 2003 the court struck down a Maryland man's death sentence based on a similar second-guessing of his attorneys.

And it was the third time this term that the court had upheld appeals by death row inmates. In March, the court abolished the death penalty for juvenile offenders; last week, the court overturned the murder conviction of a Texas man, citing prosecutors' racial bias in jury selection at his trial.

Souter's opinion also picked apart arguments made on behalf of Pennsylvania by the Bush administration, saying that the administration was mistaken to suggest that additional investigation by Rompilla's attorneys was not required because as far as the lawyers knew it was likely to be futile.

In dissent, Justice Anthony M. Kennedy, joined by Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas, said the majority was establishing an "unreasonable" new requirement for defense counsel that "if followed, often will result in less effective counsel by diverting limited defense resources from other important tasks."

But O'Connor wrote a separate concurring opinion explaining that she had provided a fifth vote to the majority based on her understanding that the ruling would not apply beyond the facts of Rompilla's case.

Yesterday's decision "simply applies our longstanding case-by-case approach to determining whether an attorney's performance was unconstitutionally deficient," she wrote.

The case is Rompilla v. Beard, No. 05-5462.