The American military has taken custody of two British detainees notorious for their roles in an Islamic State cell that tortured and killed Western hostages, removing them from a wartime prison in northern Syria run by a Kurdish-led militia, according to United States officials.
The abrupt move came as the Turkish military moved into northern Syria after getting a green light from President Trump. Turkey is targeting the American-backed Kurds — known as the Syrian Democratic Forces — who were the primary allies of the United States in the fight against the Islamic State in Syria. The Turkish invasion called into question the militia’s ability to continue securely holding some 11,000 captured ISIS fighters.
The two British men, El Shafee Elsheikh and Alexanda Kotey, were part of a four-member British cell that the Islamic State put in charge of Western hostages, who nicknamed them the “Beatles” because of their accents. Among their victims was James Foley, the American journalist who was beheaded in August 2014 for an ISIS propaganda video.
Another member of the cell, Mohammed Emwazi, or “Jihadi John,” is believed to have killed Mr. Foley. Mr. Emwazi was later killed in a drone strike.
The Justice Department has intended to eventually bring Mr. Elsheikh and Mr. Kotey to the United States for trial in Virginia, but a court fight in Britain has delayed that transfer. The lawsuit is over whether the British government may share evidence with the United States without an assurance that American prosecutors will not seek the death penalty.
The American military was taking the men to Iraq, where the United States has a base where it has held Islamic State detainees with American citizenship before transferring them to domestic soil — or, in one case, releasing a detainee in Bahrain.
It is not clear how long the two British men will stay at that base. The Justice Department has been reluctant to take custody of them and enter them into the criminal justice system — where, among other things, they will have a right to a speedy trial — until it secures the evidence still in British hands that can help support their eventual prosecution.
The British government has shared witness statements about the two men with the Justice Department, but testimony from British government officials would also probably be necessary at any trial. Mr. Elsheikh’s mother has filed a lawsuit seeking to block such cooperation because the United States government has not promised it will not seek to execute her son. Britain has abolished the death penalty.
Because of their role in abusing Americans, the two British men were at the top of a list of ISIS detainees of concern for the American government, officials said. But that list has more than five dozen names on it, including a dozen or so other Islamic State prisoners in Kurdish hands who are considered particularly dangerous.
It remains unclear whether the Trump administration will seek to take any additional detainees from the Syrian Democratic Forces as the situation in northern Syria continues to rapidly deteriorate after Mr. Trump’s decision to clear the way for Turkey to launch its operation into northern Syria.
The move is bringing to an abrupt crisis a long-simmering problem: About 50 countries have citizens in the Kurds’ prisons for ISIS fighters — and in the displaced persons camps where tens of thousands of ISIS women and children are held — and have been reluctant to repatriate them, instead leaving them in the Kurds’ hands indefinitely.
The male fighters the Kurds are holding include about 9,000 local Syrians and Iraqis, as well as 2,000 foreign fighters — including scores from Western Europe. Many European law enforcement officials fear that if they repatriate their extremist citizens, they would be unable to convict them or keep them locked up for a long time.
After Britain declined to bring Mr. Elsheikh and Mr. Kotey home for prosecution, instead stripping them of their citizenship, the United States government weighed various options for handling them itself before deciding to prosecute them in civilian court once it obtained all of the evidence it needed.
The Trump administration also weighed sending the two British men to the American military prison at Guantánamo Bay, Cuba, for a period of indefinite wartime detention without trial. Senator Lindsey Graham, Republican of South Carolina, who is a close ally of Mr. Trump’s but has criticized his Syria policy, has advocated that step.
But the military opposes getting more deeply involved in long-term detention operations, and there are steep legal obstacles to bringing the men to Cuba.
Among those challenges, transfer restrictions Congress imposed to block President Barack Obama from carrying out his plan to close the Guantánamo prison would make it illegal to transfer the men, once at the base, to domestic American soil for an eventual trial before a civilian court, and the military commissions system at Guantánamo is widely seen as too dysfunctional.
It is also not clear whether legal authority exists to hold Islamic State members — as opposed to members of Al Qaeda — in indefinite wartime detention. Once in Guantánamo, the two men would have the right to file habeas corpus lawsuits challenging the legality of their detention, raising the prospect of a ruling that the larger war effort against ISIS has been illegal.
The Washington Post earlier reported on the transfer of the detainees’ custody.
Eric Schmitt contributed reporting.