SAN QUENTIN, Calif. — Convicted killer Stanley Tookie Williams, the Crips gang co-founder whose case stirred a national debate about capital punishment versus the possibility of redemption, was executed early today.

Williams, 51, died at 12:35 a.m. Officials at San Quentin State Prison seemed to have trouble injecting the lethal mixture into his muscular arm. As they struggled to find a vein, Williams looked up repeatedly and appeared frustrated, shaking his head at supporters and other witnesses.

“You doing that right?” it sounded as if he asked one of the men with a needle.

After he was declared dead, his supporters shouted in unison: “The state of California just killed an innocent man,” as they walked out of the chamber.

The case became the state’s highest-profile execution in decades. Hollywood stars and capital punishment foes argued that Williams’ sentence should be commuted to life in prison because he had made amends by writing children’s books about the dangers of gangs and violence.

In the days leading up to the execution, state and federal courts refused to reopen his case. Monday, Gov. Arnold Schwarzenegger denied Williams’ request for clemency, suggesting that his supposed change of heart was not genuine because he had not shown any remorse for the killings.

“Is Williams’ redemption complete and sincere, or is it just a hollow promise?” Schwarzenegger wrote. “Without an apology and atonement for these senseless and brutal killings, there can be no redemption.”

Williams was condemned in 1981 for gunning down convenience store clerk Albert Owens, 26, at a 7-Eleven in Whittier and killing Yen-I Yang, 76, Tsai-Shai Chen Yang, 63, and the couple’s daughter Yu-Chin Yang Lin, 43, at the Los Angeles motel they owned. Williams claimed he was innocent.

Lora Owens, Owens’ stepmother, watched Williams die. In the days before the execution, she was one of the outspoken advocates who believed the execution should go forward. She said her stepson was shot twice in the back, even though he begged Williams for his life.

“I believe it was a just punishment long overdue,” she told ABC’s “Good Morning America.”

About 1,000 death penalty opponents and a few death penalty supporters gathered outside the prison to await the execution.

During Williams’ 24 years on death row, a Swiss legislator, college professors and others nominated him for the Nobel Prizes in peace and literature.

“There is no part of me that existed then that exists now,” Williams said during a recent interview with The Associated Press.