A divided Supreme Court on Thursday blocked the execution of a Texas killer, saying lower courts should have considered psychiatric evidence about his mental illness.

The court ruled 5-4 in the case of Scott Louis Panetti, who shot his in-laws to death 15 years ago in front of his wife and young daughter.

Lawyers for the convicted murderer say that he suffers from a severe documented illness that is the source of gross delusions.

"This argument, we hold, should have been considered" when Panetti was scheduled for execution, said Justice Anthony Kennedy, who wrote the majority opinion.

Kennedy said Panetti should have been given the opportunity to submit expert psychiatric evidence in state court because "it is uncontested" that he made a substantial showing of incompetency. Siding with Kennedy in the majority were Justices John Paul Stevens, David Souter, Ruth Bader Ginsburg and Stephen Breyer.

In dissent, Justice Clarence Thomas said that Panetti had petitioned the federal courts twice in his case and the law allows only one petition.

"The court bends over backwards to allow Panetti" to bring his current claim, despite no evidence that his condition has worsened, or even changed, since 1995, Thomas wrote. He was joined by Chief Justice John Roberts and justices Antonin Scalia and Samuel Alito.

Thomas called the majority opinion "a half-baked holding that leaves the details of the insanity standard" for the lower court to work out.

In the Panetti decision, the justices "made clear they are not talking about the broad sweep of mental illness, as some other organizations have urged," said Kent Scheidegger of the Criminal Justice Legal Foundation, which supports capital punishment.

One of Panetti's lawyers, Keith S. Hampton of Austin, Texas, said that "what this decision means is that you can bring in experts to try to determine a person's rationality."

Executing people like Panetti "serves no purpose and offends our sense of decency and common humanity," said Gregory W. Wiercioch, a staff attorney with Texas Defender Service who argued the case before the Supreme Court in April.

Texas had asked the court to reject Panetti's appeal on procedural grounds. Attorneys for the state also argued that the court should set a tougher standard for mental illness exceptions to capital punishment.

A former ranch hand and native of Hayward, Wis., Panetti had a history of mental problems including schizophrenia, recording 14 hospital stays over 11 years before his conviction. The killings took place in September 1992.

Four courts have said he was competent when he fired his trial lawyers. A jury and two courts rejected his defense of not guilty by reason of insanity. Panetti personally argued that only an insane person could prove the insanity defense, dressing in cowboy clothing and submitting an initial witness list that included Jesus Christ and John F. Kennedy.

Then-Justice Lewis Powell said 20 years ago that a person may not be put to death if he cannot perceive "the connection between his crime and his punishment."

The Eighth Amendment of the Constitution bars "the execution of a person who is so lacking in rational understanding that he cannot comprehend that he is being put to death because of the crime he was convicted of committing," they said in court papers.