Attorney General William P. Barr has notified the British government that the United States will drop its insistence on the death penalty for two admitted Islamic State members if British authorities promptly transfer evidence to aid their prosecution for suspected involvement in the executions of American, British and other foreign hostages in Syria.

In a letter Tuesday to British Home Secretary Priti Patel, Barr said London has until Oct. 15 to provide assistance or the men would be transferred to the Iraqi government for prosecution in its justice system — which human rights activists say is tantamount to a death sentence.

The men are currently detained by the U.S. military in Iraq.

“Time is of the essence,” Barr wrote. “Further delay is no longer possible” if the pair are to be tried in the United States. “Further delay is an injustice to the families of the victims.”

The Trump administration had opposed removing death as a possible sentence for El Shafee Elsheikh and Alexanda Kotey, two members of a quartet of British-raised militants implicated in the beheadings of hostages and whose accents led their captives to dub them “the Beatles.”

The case has been tied up in litigation in Britain, where Elsheikh’s mother has sued to prevent the sharing of evidence — including voice analysis tying the men to the hostages and details about their path to Syria — in a capital case. Barr’s concession — the possibility of which was first reported by The Washington Post last month — was seen as a possible means to break the impasse.

But on Wednesday, a Home Office spokesperson said in a statement: “Legal proceedings are ongoing before the Supreme Court and we are prevented by a court order from transferring the evidence to the U.S. at this time.”

It is unclear when the Supreme Court of the United Kingdom will issue its final order, which could enable the evidence-sharing. Even when it does, further litigation could ensue, Barr noted in his letter, which was first reported by Defense One.

The Beatles gained notoriety for a series of brutal executions beginning in 2014 with the beheading on camera of American journalist James Foley — the first murder of an American hostage by the terrorist group. The video of Foley, kneeling in an orange jumpsuit, went viral and brought untold heartache to his family. Several other recorded killings followed, including those of two more Americans and two Britons. In each case, the same masked executioner with a British accent wielded the blade.

That individual was identified by The Post in early 2015 as Mohammed Emwazi — better known as “Jihadi John.” He was killed in a drone strike that fall. Former hostages say the four Londoners subjected them to repeated beatings, waterboarding and mock executions.

A fourth member of the group, Aine Davis, was captured in Turkey in 2015, convicted in 2017 and is serving a seven-year prison sentence there.

Diane Foley, the mother of the slain journalist, said in an email Wednesday, “I am very grateful to hear that the attorney general has reached out to the Home Secretary.” She said she was concerned by the tight time frame but hoped that Patel would move swiftly to provide the requested evidence.

“This feels like a gift from God and our son Jim, who was brutally murdered six years ago today,” she said.

Elsheikh and Kotey were captured by Syrian Kurdish fighters in early 2018. Britain stripped the pair of their citizenship and determined that a trial in the United States, where prosecutors have a wider array of legal tools in terrorism cases — and where prison sentences are lengthier — would be the best approach.

In June 2018, then-Home Secretary Sajid Javid agreed to share evidence the British had gathered without seeking an assurance that Washington would forgo the death penalty. Elsheikh’s mother sued, seeking to halt the evidence transfer.

Following the Turkish military’s invasion of northern Syria last fall, U.S. forces moved the pair to al-Asad air base in Iraq. Defense Secretary Mark T. Esper has warned that military custody is not a long-term solution, and the hostages’ families have brought unrelenting pressure on the U.S. government to mount a prosecution.

In interviews while in Kurdish custody last summer, the two men said their role was to procure email addresses and personal information from the hostages that could be leveraged to demand ransoms.

Hostages were given “harsh treatment,” Elsheikh said. Kotey recalled Emwazi threatening to waterboard prisoners as if he had done it before. He also acknowledged interacting with Kayla Mueller, another American hostage killed in 2015.

But both men claimed they did not know hostages would be killed until Emwazi beheaded Foley on camera. “At the time there wasn’t much anyone could do about it,” Kotey said.

Letta Tayler, a senior researcher for Human Rights Watch, praised Barr for his willingness to set aside the death penalty, but criticized the threat to hand the pair to the Iraqis, whose courts, she said, have a record of convicting ISIS suspects based on confessions extracted through torture.

“The Beatles are accused of horrific crimes and should be brought to justice,” she said. “But rather than setting an ultimatum for cooperation by mid-October, the United States should do its utmost to ensure that they are prosecuted fairly, whether in the U.S. or another country that can guarantee fair trials.”

Souad Mekhennet contributed to this report.