HOUSTON (AP) — A Texas inmate faces execution Thursday for killing another prisoner more than 26 years ago, but the victim’s sister and religious leaders have asked authorities to spare his life.

William Speer, 49, is set to receive a lethal injection at the state penitentiary in Huntsville. He was condemned for strangling to death Gary Dickerson in July 1997 at the Telford state prison, located near New Boston in northeast Texas.

“I am so aware of the things that I’ve done. I’m so aware of the pain and the hurt that I’ve caused. I could just say that I’m sorry,” Speer said in a video submitted as part of his clemency petition to the Texas Board of Pardons and Paroles.

Speer’s lawyers say he has transformed while in prison, expressed regret for his actions and now helps lead a religious program that ministers to other death row inmates.

His attorneys have asked state and federal courts to halt the execution. One request for a stay focuses on allegations that prosecutors at his 2001 trial failed to disclose evidence, presented false testimony and that his trial lawyers failed to present evidence about Speer’s troubled childhood. They say Speer was physically and sexually abused as a child. Prosecutors have denied the allegations against them.

Speer’s attorneys had also asked to stop his execution over claims the state’s supply of pentobarbital, the drug used in executions, was exposed to extreme heat during a recent fire, making it unsafe. A federal judge and Texas’ top criminal appeals court this week denied appeals on this claim. A similar allegation made by another inmate, Jedidiah Murphy, was unsuccessful and he was executed earlier this month.

The Texas Attorney General’s Office said the execution drugs were tested after the fire for potency and sterility. Murphy’s execution showed the state can “handle Speer’s execution in a safe and humane manner,” authorities said.

At the time of inmate Dickerson’s killing, Speer had been serving a life sentence for fatally shooting a friend’s father, Jerry Collins, at the man’s Houston area home. Speer was 16 then.

The paroles board on Tuesday voted 7-0 against commuting Speer’s death sentence to a lesser penalty. Members also rejected granting a six-month reprieve.

Speer killed Dickerson in a bid to join the Texas Mafia prison gang, prosecutors said. The gang ordered the hit after mistakenly concluding Dickerson had informed authorities about tobacco it had tried to smuggle into the prison.

Speer and another inmate, Anibal Canales Jr. were sentenced to death for the killing. Canales remains on death row.

At Speer’s trial, Sammie Martin, who is Dickerson’s only living sibling, told jurors her mother was devasted by her brother’s death.

But Martin has now asked that Speer’s life be spared.

“I have spent much time reflecting on what justice my brother and my family deserved,” Martin wrote in federal court documents filed this week. “In my heart, I feel that he is not only remorseful for his actions but has been doing good works for others and has something left to offer the world.”

Martin said she was never informed by prosecutors about Speer’s scheduled execution.

In court documents filed this week, lawyers with the Texas Attorney General’s Office said that despite Martin’s feelings about Speer’s execution, “the state retains its interest in deterring gang murders and prison violence, as well as seeing justice done for Dickerson.”

A group of religious leaders from around the country have also asked that Speer be spared. In a letter to the paroles board and Gov. Greg Abbott, they wrote that Speer’s religious work with other prisoners “does not excuse his actions, but it gives us a fuller picture of who Will is as a human, Christian, leader, and teacher.”

Speer would be the seventh inmate in Texas and the 21st in the U.S. put to death this year.

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