Amid recent reports detailing the shocking, systemic sexual abuse of IS captives, counter-terrorism officials say that American hostage Kayla Mueller was raped multiple times by the terrorist leader, before her death earlier this year.

Last February, the self-proclaimed Islamic State terrorist group claimed that Mueller, held captive since 2013, had been killed in a Jordanian airstrike. These claims were later confirmed by US officials.

"We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life," read a statement from Mueller’s family.

© AP Photo / Militant video Leader of the Islamic State group Abu Bakr al-Baghdadi

Now the family has come forward with more details about their daughter’s life in captivity. According to US counter-terrorism officials, Mueller was sexually assaulted multiple times by Islamic State leader, Abu Bakr al-Baghdadi.

"We were told Kayla was tortured, that she was the property of al-Baghdadi," the family told ABC News on Friday. "We were told that in June by the government."

Earlier reports had suggested that Mueller had been forced into marriage with Abu Sayyaf, an upper level IS member in charge of maintaining the group’s oil and gas revenues. There was even early speculation that she decided to cooperate willingly with the terrorist organization.

"[I am] completely unharmed + healthy (put on weight in fact); I have been treated w/ the utmost respect + kindness," read a letter smuggled to her family nearly one year after her capture.

© REUTERS / Mueller Family/Handout via Reuters Kayla Mueller (L), 26, an American humanitarian worker from Prescott, Arizona is pictured with her mother Marsha Mueller in this undated handout photo obtained by Reuters February 6, 2015

But the new evidence suggests that Mueller, who traveled to Syria as an aid worker, was taken to Sayyaf’s home by al-Baghdadi shortly after her kidnapping.

"Baghdadi was at the house of Sayyaf," a US official told ABC. "He delivered Kayla Mueller, live and in person."

The IS leader would then sexually assault Mueller during his regular visits to Sayyaf’s compound.

The evidence for the 26-year-old’s treatment comes from multiple sources, not least of which are a pair of Yezidi girls who recently escaped captivity from the Sayyaf compound.

The girls, 16 and 18, were themselves part of a massive slave trade being conducted by the Islamic State. Detailed in depth by a Thursday New York Times report, the terrorist group has been kidnapping women and girls as young as 12, and selling them as sex slaves.

The trade, operating on a massive scale, is organized by Sayyaf’s wife, Umm Sayyaf.

Those abductions are focused primarily on the Yazidi religious minority, which the Islamic State deems especially deviant because of its polytheistic beliefs. Roughly 5,270 Yazidi women and girls were kidnapped last year, alone. Women are sold; men are rounded up and shot.

Multiple girls who have managed to escape have told shocking accounts of how the IS leadership views the rape of these women and children as an act of religious worship.

"Every time that he came to rape me, he would pray," a 15-year-old girl told the New York Times. "He said that raping me is his prayer to God. I said to him, 'What you’re doing to me is wrong, and it will not bring you closer to God.' And he said, 'No, it’s allowed. It’s halal.'"

© AP Photo / Felicia Fonseca Memorial for Kayla Mueller in her hometown of Prescott, Arizona

The captive girls also described Mueller’s presence in Sayyaf’s home. According to their accounts, they shared a room with the American hostage, and Mueller would often put herself at risk to protect the others.

They also confirm the accounts of al-Baghdadi’s visits.

"I saw him often, he came there often," one of the Yazidi girls said, according to the Independent. "And every time he came, he took Kayla away. She would always come back after."

Upon returning, Mueller would tell them what had happened, often crying.

"He had married her by force," one of the girls said.

Mueller traveled to Syria to help aid refugees, and was working with humanitarian groups Support to Life and Spanish Doctors Without Borders when she was kidnapped.

"I remember mom always telling me that all in all in the end the only one you really have is God. I have come to a place in experience where, in every sense of the word, I have surrendered myself to our creator b/c literally there was no else," she wrote in a letter shortly before her death.

"+ God + by your prayers I have felt tenderly cradled in freefall."