LOS ANGELES (AP) — A man who has been on California's death row for more than three decades can't be executed because he's intellectually disabled, the state's highest court ruled Thursday.

In a unanimous decision, the California Supreme Court overturned the death sentence of Robert Lewis Jr., citing "substantial evidence" that the 65-year-old is intellectually disabled, including having low IQ scores and being illiterate.

Lewis was convicted of murder and robbery in the stabbing and shooting death of Milton Estell in Long Beach in October 1983.

Prosecutors had argued that Lewis should remain on death row, saying he managed to successfully gamble in Las Vegas, bantered with police during questioning and has "street smarts."

The court rejected those arguments, citing findings by a lower court judge that Lewis "was not a successful street hustler, having been arrested and convicted numerous times, nor was he adept at deceiving police during his interview."

Jurors deciding Lewis' fate didn't hear evidence of his intellectual disabilities. In 2002, the U.S. Supreme Court ruled that states should not execute the intellectually disabled, citing cruel and unusual punishment.

Lewis' attorney, Robert Sanger, said his client's sentence will be reduced to life in prison without the possibility of parole.

Sanger, who has argued Lewis' case since 1994, had also sought a new trial for his client but was unsuccessful.

He had argued that Lewis' trial attorney was ineffective and criticized the unusually short length of his trial — just four days. The penalty phase that led to Lewis' death sentence took just over 90 minutes, he said.

"It was a total mess," Sanger said. "The trial was extremely unfair."

Although Sanger believes Lewis deserved a new trial, he said the court's decision overturning his death sentence was a victory.

"I'm very pleased that we can say this: The state is not going to kill Robert Lewis," Sanger said. "It would have been an absolute shame."