Attorney General William Barr notified the British government this week that the U.S. will not impose the death penalty on two Islamic State militants accused of helping savagely kill American hostages—a bid by the Trump administration to bring the men for trial in U.S. criminal courts using evidence currently held by the U.K.

Even as he waived capital punishment, Barr gave Britain an ultimatum: Provide the evidence by October 15, or the men will be turned over to Iraqi authorities, where they would face almost certain execution.

“Time is of the essence,” Barr wrote in a letter to British Home Secretary Priti Patel made public Thursday, and first reported by the web site Defense One.

Barr added that “further delay is no longer possible” if the pair are to be prosecuted in the United States and “further delay is an injustice to the families of the victims.”

American officials say Britain holds crucial evidence needed to try the men, Alexanda Kotey and El Shafee Elsheikh, British born-ISIS militants who were part of a group dubbed “the Beatles” that allegedly terrorized and killed American and British captives in Syria.

But, as Barr noted in the letter, the U.K. Supreme Court has ruled that the British government could not provide evidence against the two men as long as the death penalty remained a possibility. The men are currently being held in U.S. military custody in Iraq.

As Barr also noted, the U.K. Supreme Court has yet to issue a final order, which the U.K. government has said prevents it from turning over the evidence. It’s unclear when that order might come.

A U.S. official briefed on the matter told NBC News that the men will not be transferred to the U.S. until the classified British intelligence has been turned over.

“They want an open and shut case before they transfer them to DOJ custody,” the official said. “They don’t want any room for error.”

Barr’s letter was released roughly two weeks after NBC News reported that he told families of the victims he would be ruling out the death penalty to ensure that the British government can provide evidence against the ISIS detainees.

In interviews last year broadcast recently by NBC News, the men further incriminated themselves in the mistreatment of Western hostages in Syria, including Americans Kayla Mueller and James Foley.

The pair for the first time admitted their involvement in the captivity of Mueller, an aid worker who was tortured and sexually abused before her death in 2015.

“She was in a room by herself that no one would go in,” Kotey said.

Elsheikh added more detail, saying, “I took an email from her myself,” meaning he got an email address the Islamic State militant group could use to demand ransom from the family. “She was in a large room, it was dark, and she was alone, and … she was very scared.”

In one email reviewed by NBC News, ISIS demanded the Muellers pay 5 million euros and threatened that if the demands weren’t met, they would send the family “a picture of Kayla’s dead body.”

Elsheikh also implicated himself in the abuse of American James Foley. “I didn’t choke Jim,” he said. “If I choked Jim I would say I choked him. I mean, I’ve — I’ve hit him before. I’ve hit most of the prisoners before.”

Elsheikh said that sometimes Foley would let himself become a target to make sure hostages got enough food. “If the guard would ask, ‘Is the food enough?’ some of the other prisoners were very timid,” Elsheikh said. “It was always him who would say, ‘It’s not enough,’ and take the risk of retaliation from guards.”

The families of American hostages murdered by ISIS have been urging the Trump administration to try them in a U.S. civilian court.

“They did so much horror to so many people,” Kayla’s mother, Marsha Mueller, told NBC News last month. “They need to be brought here. They need to be prosecuted. The other thing that’s really important to me about this is I need information about Kayla. We know so little about what happened to her.”

“I think they should be held accountable,” Foley’s mother, Diane, previously told NBC News. “They definitely should go on trial.”