Islamic State leader Abu Bakr al Baghdadi repeatedly raped American hostage Kayla Mueller, according to accounts provided to US intelligence officials.
According to a Yazidi teenager who was held with 26-year-old Ms Mueller, al Baghdadi took the American as his “wife”, repeatedly forcing her to have sex when he visited.
Mueller family spokeswoman Emily Lenzner told the Associated Press news agency the 14-year-old Yazidi girl, who later escaped, spoke to US officials.
They checked out her story with other intelligence before passing it on in June to Kayla Mueller’s parents, Carl and Marsha Mueller, Ms Lenzner added.
An unnamed US official confirmed the account, which was first reported by London’s Independent.
According to the newspaper, Ms Mueller was kept in a room with four Yazidi teenagers, whom she tried to protect from abuse by Umm Sayyaf and militants.
The girls are said to have regarded Ms Mueller as a “mother figure”.
The American aid worker would leave the room when al Baghdadi visited and weep afterwards as she told the Yazidi girls about the sexual abuse she had suffered, reports the newspaper.
Abu Sayyaf, a Tunisian, was killed in a US special forces raid on his Syrian compound in June, which American officials said reaped vital intelligence about the Islamic State group.
Umm Sayyaf, who was detained during the raid, is said to have co-operated with US interrogators and has been turned over to the Iraqi Kurds for trial.
Ms Mueller, from Prescott, Arizona, was taken prisoner in Aleppo, Syria, in August 2013.
The Islamic State group said she died when Jordanian warplanes struck the building where she was being held, though they offered little proof for the claim.
The Mueller family released a heart-wrenching letter she wrote to them from captivity in which she said she had been “treated w/ the utmost respect + kindness”.
“Do not fear for me,” she said in the letter, widely thought to have been written under duress.