A trial witness has acknowledged giving false testimony that a Georgia inmate confessed to being the triggerman in a 1996 shotgun slaying for which he is to be executed this week, the inmate's attorneys said in court documents Tuesday.

Robert Butts Jr., 40, is scheduled to receive a lethal injection Thursday. Butts and co-defendant Marion Wilson were both sentenced to death after being convicted of murder and armed robbery in the slaying of 24-year-old Donovan Corey Parks.

Butts' attorneys are asking the state Board of Pardons and Paroles to spare his life, and the board released Butts' 54-page petition the day before his scheduled clemency hearing Wednesday.

"I think about Mr. Parks and that night every single day, going over it again and again in my mind," Butts said in a statement included with his petition. "There's no excuse for what I did, and I'm tremendously sorry for what happened to Mr. Parks."

Prosecutors say Butts and Wilson targeted Parks after standing behind him in the checkout line of a Walmart store in the central Georgia city of Milledgeville. They asked Parks for a ride, and he agreed.

A short distance from the store, one of the men showed Parks a shotgun and ordered him to pull over. Parks was dragged from the car and killed by a single shot to the head. Investigators later found a shotgun under Wilson's bed. A witness said Butts had given the shotgun to Wilson to hold after the killing.

In the document, Butts' attorneys insist he wasn't the shooter and point to a jailhouse witness who now says his trial testimony was a fabrication.

Horace May told jurors that he and Butts were behind bars together when Butts confessed to being the one who shot Parks. Butts' lawyers wrote that May has now signed an affidavit saying he made up the story out of sympathy for Wilson, whom he also met in jail.

"He asked me to do it for his kid, and I agreed," May told Butts' attorneys, according to his clemency petition.

The document also quotes May saying he was told by Wilson that "he and Robert had agreed to steal Donovan's car, but he had told Robert that they would release Donovan."

Another inmate witness who implicated Butts as the shooter at his trial, Gary Garza, refused to speak with Butts' attorneys, the petition said. But the lawyers wrote that May's recantation of his trial testimony "alone is reason for this Board to exercise its awesome powers and spare Robert Jr.'s life."

Regardless of who pulled the trigger, juries in separate trials found sufficient evidence to sentence both men to death because Parks was killed during the commission of an aggravating felony, armed robbery. Wilson's case is still pending in the courts.

Butts' attorneys argue in his clemency petition that the single aggravating factor wouldn't warrant a death sentence in Georgia today. They also ask the board to consider commuting Butts' sentence to life in prison after weighing abuse and neglect during Butts' childhood, the fact that he was just 18 when the crime occurred and that he has expressed remorse.