RICHMOND -- Gov. Tim Kaine on Monday commuted the death sentence of a Danville man who committed three murders, determining that Percy L. Walton is too mentally impaired to understand that he was about to be executed.

Kaine announced his decision one day before Walton, 29, was to be executed by lethal injection for robbing and killing three neighbors in 1996. The governor commuted Walton's sentence to life in prison without possibility for parole.

It is the first time that Kaine has commuted a death sentence since taking office in 2006. The governor has allowed five executions to proceed, despite his long-standing objection to the death penalty.

Walton's execution already had been delayed three times because of questions about his competence. His lawyers have argued that the inmate is so mentally impaired that he did not understand why he was sentenced to death or that he would die for his crimes.

A 1986 U.S. Supreme Court ruling forbids the execution of inmates who are unaware of the punishment they are about to receive and why.

Walton was sentenced to death for the 1996 killings of Jessie E. Kendrick and Elizabeth W. Kendrick, an elderly couple, and 33-year-old Archie D. Moore Jr.

A federal court delayed the execution in 2003 but later determined that Walton was mentally competent. Walton lost an appeal on a 7-6 decision in the U.S. Court of Appeals for the Fourth Circuit.

Kaine twice postponed the execution in 2006 to allow for further evaluation of Walton's mental health. A clemency petition "presented significant evidence that Walton had schizophrenia ... and that Walton's mental state had deteriorated since 2003," Kaine said in a statement issued Monday.

Between June and December of 2006, Kaine obtained more information about Walton's condition, including Department of Corrections records, psychiatric evaluations and accounts from people who interacted with Walton on a regular basis. The governor decided in December 2006 that Walton met the Supreme Court's definition of mental incompetence and delayed the execution by 18 months.

"Over the course of those 18 months, there has been no discernible improvement in Walton's condition and no evidence that his mental impairment is temporary," Kaine said.

Kaine described Walton's behavior in detail and said the inmate "lives in a self-imposed state of isolation that includes virtually no interest in receiving or understanding information."

Kaine said commutation of the death sentence "is now the only constitutionally appropriate course of action."

Attorney General Bob McDonnell, a Republican, said he disagreed with the Democratic governor's decision. McDonnell said "multiple courts" have considered evidence of Walton's mental capacity and should have already considered any new evidence.

"Nothing has prevented Walton from bringing such evidence to the federal courts for further review in light of the U.S. constitutional standard," McDonnell said in a written statement. "Evidence of an inmate's competency is more effectively evaluated by a judicial officer. Thus I respectfully disagree with the governor's decision that clemency is now warranted in this case."

Nash Bilisoly, a lawyer for Walton, said Kaine "acted appropriately." But Walton won't understand the impact of the governor's decision, Bilisoly said.

"I can assure you he won't," Bilisoly said. "I won't even go tell him. I don't think he would understand any of it."

Kaine said he was "mindful of terrible injustice that Walton perpetrated" against the victims and that his thoughts were with the victims' families.

Irene Jurscaga, the sister of Elizabeth Kendrick, told The Associated Press that she was disappointed by Kaine's decision.

"It's just another person that our tax dollars have to feed," said Jurscaga. "He isn't deserving to be alive, someone who created such a heinous crime."

The Associated Press contributed to this report.