This file image, made from video posted on a militant Web site in 2014, shows the leader of the Islamic State, Abu Bakr al-Baghdadi. (Uncredited/AP)

The leader of the Islamic State personally kept a 26-year-old American woman as a hostage and raped her repeatedly, according to U.S. officials and her family.

The family of Kayla Mueller said in an interview Friday that the FBI had informed them that Abu Bakr al-Baghdadi, the emir of the self-proclaimed Islamic State, had sexually abused their daughter, a humanitarian worker.

Mueller’s parents said the FBI first spoke to the family about the sexual assault in late June and provided more details two weeks ago. The bureau pieced together what happened to the American from interviews with other hostages and the captured wife of a senior Islamic State figure.

The FBI also told the Muellers that their daughter had been tortured.

“June was hard for me,” said Marsha Mueller, Kayla’s mother. “I was really upset with what I heard.”

The disclosure that Mueller was raped by Baghdadi adds to the grim evidence that the exploitation and abuse of women has been sanctioned at the highest levels of the Islamic State. The sexual enslavement of even teenage girls is seen as religiously endorsed by the group and regarded as a recruiting tool.

News of Baghdadi’s abuse of Mueller, who was from Prescott, Ariz., was first reported Friday by the Independent, a London newspaper.

“As painful as this is for our family, we just feel like the world needs to know the truth,” said Carl Mueller, Kayla’s father. The Muellers noted that Friday would have been their daughter’s 27th birthday.

The Islamic State claimed that Mueller was killed earlier this year after a Jordanian fighter plane dropped a bomb on the building where she was being held. The U.S. government confirmed the death but not the cause.

Mueller’s family had previously released a letter their daughter had written in which she talked about the conditions of her captivity. “Please know I am in a safe location, completely unharmed + healthy (put on weight in fact); I have been treated w/the utmost respect + kindness,” she wrote in the letter, which the family received in the spring of 2014.

Kayla’s mother said she had thought her daughter had been treated reasonably until she learned about the conditions of her captivity during a June meeting with FBI officials in Washington. The FBI said they learned about Mueller’s mistreatment from the wife of a senior Islamic State operative captured earlier this year, as well as young female members of the Yazidi religious sect who had spent two months in captivity with Mueller before at least one of them escaped last fall.

U.S. officials had previously said that Mueller was abused by her captors, but it was not known until now that she was kept as a sex slave of the leader of the Islamic State.

Baghdadi is a former Iraqi insurgent who was detained by U.S. forces early in the Iraq war. He was part of an al-Qaeda affiliate in Iraq that was thought to have been largely destroyed before the civil war in Syria allowed it to regenerate.

Though little is known about his background, Baghdadi is regarded as an experienced fighter and a capable leader. His most prominent public appearance came last year when he surfaced at a mosque in Mosul to declare himself the leader of a restored caliphate.

Mueller was abducted in August 2013 after leaving a hospital in the Syrian city of Aleppo. Three months after she died, the compound where she had been held was targeted in a raid by U.S. Special Operations forces.

The operation was aimed at capturing Abu Sayyaf, the nom de guerre of a high-ranking Tunisian member of the Islamic State, who was thought to be in charge of oil smuggling and other illicit enterprises that have funded the terrorist group.

Sayyaf was killed in what U.S. officials described as intense “close quarters combat.” But his wife, identified only as Umm Sayyaf, survived and was eventually brought back to Iraq aboard a bullet-riddled U.S. aircraft. She was then questioned by U.S. interrogators for months, providing information about Mueller as well as the Islamic State’s leadership, before recently being turned over to Iraqi custody.

Systematic abuse

Mueller’s mistreatment is the latest evidence of the Islamic State’s systematic abuse of women on a significant scale.

A report released in April by Human Rights Watch accused the Islamic State of war crimes for its brutal treatment of female Yazidis — many of them teenagers — who were captured in Iraq last August, taken to Syria and forced into sexual slavery by the Islamic State.

After surging into the northern Iraqi town of Sinjar last year, Islamic State fighters captured as many as 1,000 Yazidi women, many of whom were given a bleak choice of “marriage” to a fighter or imprisonment and potential death.

The Human Rights Watch report focused on 20 women who escaped the group and provided detailed accounts of their treatment.

One described attempting to kill herself by going into a bathroom, turning on water and grasping a wire “to electrocute myself but there was no electricity.”

After being discovered, she said she was badly beaten, handcuffed to a sink, stripped of her clothes and washed. “They took me out of the bathroom, brought in [a friend] and raped her in the room in front of me,” said the woman, who is referred to only as Leila. Later she, too, was raped.

Another victim, who was only 12 years old, said that after being abducted in Sinjar, the women in her family were separated from the men and sent to a house in Mosul. Islamic State fighters “would come and select us,” she said. One of the captors beat her, she said, and then “spent three days having sex with me.”

A recent issue of the English-language magazine published by the Islamic State described the taking of sex slaves as religiously justified. The article — titled “Slave girls or prostitutes?” — endorsed the practice, saying sex slaves are “lawful for the one who ends up possessing them even without pronouncement of divorce by their [non-Muslim] husbands.”

The article went on to cite accounts that the prophet Muhammad “took four slave-girls as concubines,” a purported religious basis for the practice.