He added that the treatment Mr. Kotey received in the United States stood in “stark contrast to the cruelty, inhumanity and indiscriminate violence touted by the terrorist organization he espoused.”
Prosecutors said Mr. Kotey participated in the American prisoners’ “seizure, detention and hostage negotiations” and also took part in hostage operations involving “British, Italian, Danish and German nationals, among others.”
The hostages were abused badly in Syria. They were subjected to waterboarding, mock executions, painful stress positions, food deprivation, beatings with sticks, chokeholds causing blackouts and electric shocks.
Their jailers forced the hostages to fight each other and to witness murders, the Justice Department has previously said. In some cases, hostages were released after ransoms were paid. The American hostages met gruesome deaths that rattled the Obama administration and forced the government to revamp the way it dealt with the families of hostages. According to prosecutors, Mr. Kotey said the violence was part of “controlling” the hostages.
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, as well as Mr. Kassig, an aid worker. Mr. Kotey said he traveled to Syria in August 2012 with Mr. Emwazi and first joined a Qaeda offshoot, Al-Nusra Front, before switching to ISIS.
The fourth British member of the group, Aine Davis, has been imprisoned in Turkey on terrorism charges. Mr. Kotey’s cooperation would be an important part of any effort to prosecute Mr. Davis if Turkey agreed to extradite him to the United States or Britain.
Mr. Kotey said he was not present when the Americans were beheaded on video but was aware beforehand that ISIS intended to kill them. When he begins providing information to the Americans, the British and other foreign governments, it is not clear if he will be able to provide any reliable details about where the Americans were buried or what happened to Ms. Mueller, a young woman from Arizona who was imprisoned with the other Americans and hostages.