PHILADELPHIA—A Philadelphia judge has granted a new evidence hearing for a killer set to be executed next month, saying she wants to hear from his co-defendant and the trial prosecutor.

Terrance "Terry" Williams admits he killed Amos Norwood with a tire iron at age 18, months after killing another man. Williams now says the 56-year-old Norwood had been sexually abusing him since he was 13.

In a new affidavit, co-defendant Marc Draper says he told detectives and a prosecutor that Williams "snapped" over his relationship with Norwood. The defense believes that corroborates Williams' claims of abuse—which the jury did not hear.

Common Pleas Judge R. Teresa Sarmina on Friday ordered Draper and the trial prosecutor, Andrea Foulkes, to testify at a hearing Thursday.

Draper said Foulkes and city detectives told him not to mention the relationship when he testified against Williams, but to instead focus on the alleged robbery motive. The pair stole cash and credit cards from Norwood during the 1984 slaying at a cemetery, then went gambling in Atlantic City, N.J.

"Neither Ms. Foulkes nor the police wanted to hear anything about the case having to do with Norwood having sex with Terry," Draper, who is serving a life sentence for his role in the slaying, wrote in an affidavit signed Tuesday. "They did not want me to say the case involved a relationship. They wanted me to say it was only a robbery. Ms. Foulkes made it clear to me that I had to stick to the story."

Foulkes, now a federal prosecutor in Philadelphia, said she cannot comment on the pending litigation.

Williams, 46, of Philadelphia, is set to be executed on Oct. 3. He would be the first person executed in Pennsylvania since 1999. He claims that both men he killed had molested him.

He has exhausted his appeals, but could win a reprieve if he can prove the government withheld evidence or otherwise interfered with his 1986 trial, public defender Shawn Nolan said. Sarmina, referring to the alleged abuse, has said she thinks juries are more likely to give a life sentence when the victim is not sympathetic.

Norwood's widow, several jurors and death-penalty opponents, including Roman Catholic Archbishop Charles Chaput of Philadelphia, are among those urging that Williams' life be spared.

On a separate legal track, Williams met with the state Board of Pardons on Thursday, before a clemency hearing Monday.

The Philadelphia district attorney's office called the Draper affidavit nothing new and continued to oppose efforts to delay the execution.

"Terrance Williams is once again trying to use hearsay allegations to escape the consequences of his deadly actions 28 years ago. None of these claims are new," the office said in a statement.

Draper, a policeman's son, said he stuck to his story for many years but is coming clean now partly because of his spiritual development.

"If what I really knew about Terry and Mr. Norwood and what happened that night can help him, so be it," he wrote. "I want the truth to come out now, before Terry Williams is executed."

There are 200 people on death row in Pennsylvania, but the state has executed only three people since the U.S. Supreme Court restored the death penalty in 1976. All three chose to end their appeals.