WASHINGTON — The British government must withhold key evidence from the United States for the trial of two Islamic State detainees because the Trump administration has not provided assurances that the men will not be executed, the British Supreme Court ruled on Wednesday.
The detainees were half of a cell of four ISIS Britons who handled Western hostages — some of whom were eventually beheaded on propaganda videos — and whose victims nicknamed them the Beatles because of their accents. Captured by a Kurdish militia in Syria in early 2018, the detainees, El Shafee Elsheikh and Alexanda Kotey, are being held by the American military in Iraq.
The ruling in a lawsuit was a major setback for senior law-enforcement officials in both countries. The British government had stripped the two men of their citizenship and had agreed to share evidence about them for use in an American trial without assurances that they would be not face capital punishment, even though Britain has abolished the death penalty.
“No further assistance should be given for the purpose of any proceedings” against the men “in the United States of America without the appropriate death penalty assurances,” Justice Brian Kerr wrote.
The fate of the two detainees has been a fraught question since an American-backed Kurdish militia, the Syrian Democratic Forces, captured them. In October, when the Turkish military moved into northern Syria after getting a green light from President Trump, the American military took custody of them from a Kurdish prison and has been holding them in Iraq.
“This is a big deal,” said Robert M. Chesney, a University of Texas national-security law professor. “The decision is a tremendous blow to the U.S. government’s plan to prosecute the Beatles in an American court. Of course, the problem could be solved by focusing on a life sentence and giving up on the death penalty. But that is a bitter pill to swallow given the enormity of their crimes.”
Indeed, the ruling raised the question of whether Attorney General William P. Barr would change the stance insisted upon by his predecessor, Jeff Sessions, and agree that the maximum penalty that American prosecutors would seek against the two men was life in prison. That would resolve the problem keeping British officials from sharing the key evidence.
Marc Raimondi, a Justice Department spokesman, expressed disappointment with the ruling and said law enforcement officials were considering their next move.
“As our investigation of these individuals continues, we will work with our U.K. counterparts on a path forward, consistent with our shared commitment to ensuring that those who commit acts of terror are held accountable for their crimes,” he said in a statement.
The evidence the British gathered was crucial to any trial, former American law enforcement officials said. The men are said to have covered their faces while interacting with the hostages. To strengthen the case beyond a reasonable doubt that they are indeed two of the four Beatles, prosecutors need supplementary material gathered by Britain about their backgrounds, associations, radicalization, movements and activities.
British officials had given in to American demands that its prosecutors receive the evidence without strings attached in part because the British feared that the alternative was that the Trump administration would send the two Britons to the American military prison at Guantánamo Bay, Cuba.
Families of victims of the Beatles have opposed that idea both as a matter of symbolism — ISIS dressed Western hostages in Guantánamo-style orange jumpsuits before murdering them, including James Foley, the American journalist who was beheaded in August 2014 — and as a matter of substance. The military commissions system at the base has proved to be dysfunctional and incapable of getting contested cases swiftly to trial.
In a brief interview, Mr. Foley’s mother, Diane Foley, urged the Trump administration to prosecute the men in federal court even without the death penalty. “Not prosecuting them is immoral,” she said. “They need to be held accountable for their horrible crimes.”
Ms. Foley said she opposed the death penalty for the detainees, adding that it would only “fulfill their desire for martyrdom.”
The British government has also balked at sharing evidence if the men are sent to Guantánamo, raising the prospect of indefinite detention without a trial. Moreover, transferring them there would set up a court battle that executive branch officials want to avoid: whether the government’s claim is valid that Congress’s 2001 authorization to fight Al Qaeda also covers the use of armed force against the Islamic State in Syria.
British officials initially sought assurances from the United States that it would not seek the death penalty in exchange for providing the evidence, as is routinely the case. But the Trump administration refused to grant them. American law enforcement officials wanted to seek capital punishment and argued that since the British government essentially was not handling its own mess, it should not place restrictions on the United States.
Eventually, the British government gave in and agreed to share the evidence with the United States without death penalty assurances. But the mother of Mr. Elsheikh sued. A lower-court judge threw out that case, and British officials had repeatedly told their American counterparts that they were confident they would win the appeal as well.
But the seven justices on the British Supreme Court unanimously ruled that the British home secretary’s decision to transfer personal data to law enforcement authorities abroad for use in capital criminal proceedings without any safeguards violated a data protection law passed in 2018.
The justices struck down the decision on narrow grounds — that the British government had failed to perform a legally required assessment about whether sharing the personal data was necessary and complied with certain standards. But multiple justices also thought doing so would have been illegal regardless as a violation of the suspects’ rights.
The two men have given numerous interviews while imprisoned, at first striking an unrepentant tone and dodging questions about any personal culpability in the killing of the hostages. But as time passed, the men seemed more apologetic and admitted ransoming Western hostages.
One of the four, Mohammed Emwazi, was killed in an airstrike in 2015 in Syria. Known as Jihadi John, he is believed to have personally beheaded American and British hostages. A fourth, Aine Davis, is imprisoned in Turkey on terrorism charges. The extradition of Mr. Davis to the United States seems unlikely as the American-Turkish relationship continues to deteriorate.
The British extremists were known for their brutality. They repeatedly beat the hostages they kept imprisoned in Raqqa, Syria, formerly the Islamic State’s self-declared capital, and subjected them to waterboarding and mock executions.
In addition to killing Mr. Foley, Mr. Emwazi was believed to have killed the American journalist Steven J. Sotloff as well as Abdul-Rahman Kassig, an aid worker. The American government says the group beheaded more than 27 hostages.
The constellation of opinions in the British Supreme Court case offers no sympathy for the two detainees. “It is difficult to imagine more horrific murders than those which Mr. Elsheikh and Mr. Kotey are alleged to have carried out,” Justice Kerr wrote, calling their alleged crimes “monstrous” and “heinous.”
If brought to the United States, the men have been expected to face charges in federal court in Northern Virginia. Any trial would probably involve former hostages, especially from Italy, France, Spain and Denmark, testifying and recounting the horrors they experienced while imprisoned by the Islamic State in Syria.
All four men had lived in West London. Mr. Kotey, born in London, is of Ghanaian and Greek Cypriot background, while Mr. Elsheikh’s family fled Sudan in the 1990s. Both men have been designated foreign terrorists by the United States.