Another member of the cell — Mohammed Emwazi, better known as Jihadi John — was killed in an airstrike in 2015 in Syria. Mr. Emwazi was believed to have killed Mr. Foley and Mr. Sotloff, both Americans, as well as Peter Kassig, an aid worker.
A fourth man, 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 deteriorates.
The families of Mr. Foley and Mr. Sotloff as well as those of Kayla Mueller and Peter Kassig, two Americans who were also killed in Syria, said they welcomed the news the two men were being brought to the United States to be prosecuted.
“James, Peter, Kayla and Steven were kidnapped, tortured, beaten, starved, and murdered by members of the Islamic State in Syria,” the families said in a statement. “Now our families can pursue accountability for these crimes against our children in a U.S. court.”
The families added they were particularly grateful to Mr. Barr for his decision to waive the death penalty against the two men and thanked Ali Soufan, a former F.B.I. agent who quietly helped them.
Any trial would most likely involve former hostages, especially from Italy, France, Spain and Denmark, possibly testifying and recounting the horrors they experienced while imprisoned by the Islamic State in Syria.
The two men helped supervise detention facilities where hostages were held, coordinated ransom negotiations for their release and engaged in a prolonged pattern of physical and psychological violence against their prisoners, charging documents said. For example, on April 25, 2014, prosecutors said, the men forced prisoners from Europe to witness the murder of a Syrian man by Mr. Emwazi as part of a hostage negotiation process.