While the British government showed the evidence it had to American officials, it was unable to transfer it because Mr. Elsheikh’s mother filed a lawsuit challenging the legality of further cooperation. The primary issue was that the attorney general at the time, Jeff Sessions, opted not to make assurances that the United States would forgo the death penalty — over the objections of four American families whose children were killed by the Islamic State.
Because Britain has abolished the death penalty, such assurances are usually routine when the United States is working with the British criminal justice system. In March, the British Supreme Court preliminarily sided with Mr. Elsheikh’s mother and blocked the government from sharing the evidence and providing assistance in the case.
But last month, after a push by the Pentagon to find a disposition option for the detainees, Attorney General William P. Barr reversed Mr. Sessions’s stance and provided assurances to Britain that American prosecutors would not seek the death penalty against the two potential defendants after all.
At the same time, however, Mr. Barr imposed a deadline of Oct. 15 for the British government to resolve litigation that has tied up its ability to transfer the evidence. If it failed to do so by then, he threatened, the United States would instead transfer the two men to the custody of the Iraqi government.
The concession, coupled with that threatened deadline, appears to have cleared the logjam in Britain. While Mr. Elsheikh’s mother then filed a new case before the High Court saying it would be illegal to transfer the evidence because her son should be tried in Britain, a lower body in the British judicial system quickly disposed of the case in its ruling on Tuesday.
When the British government had previously proposed to share evidence “without a death penalty assurance, this was an unusual case,” wrote one of the judges, Dame Victoria Sharp. “Notwithstanding the barbaric nature of the offenses alleged, now that the death penalty assurance has been given it has become” routine and the government can proceed.
Senator Jeanne Shaheen, Democrat of New Hampshire, who has worked closely with the Foley family and the Justice Department on trying to get a civilian trial for the two men, praised the development in a statement as a “breakthrough.”