ISIS leader Abu Bakr al-Baghdadi
ISIS leader Abu Bakr al-BaghdadiReuters

The head of ISIS reportedly kept a 26-year-old woman from the US as a hostage and sex slave, according to both US officials and the victim's family.

Kayla Mueller's family told the London-based Independent that the FBI informed them about Kayla's abuse and that she had been tortured. The Bureau said it had obtained its information by interviewing other hostages and the wife of another senior ISIS member.

The fact that Abu Bakr al-Baghdadi himself treated Mueller this way suggests that ISIS's frequently-reported abuse and rape of women is endorsed on all levels.

Human Rights Watch released a report several month ago in which it accused ISIS of perpetrating war crimes in its treatment of female Yazidi prisoners, many of whom were forced to serve as sexual slaves for fighters.

ISIS has even published a magazine in English, in an article entitled "Slave Girls or Prostitutes" argues that Islam permits raping captives, pointing out that Muhammad "took four slave-girls as concubines."

Mueller was taken captive in August 2013 as she left a hospital in Aleppo, Syria. ISIS announced that she was killed earlier this year. The US government has confirmed Mueller's death, but not the jihadist organization's claim that she died due to a Jordanian bomb that was dropped on her building.