WASHINGTON — After the Islamic State declared that Kayla Mueller, a 26-year-old humanitarian worker from Arizona it was holding hostage, was killed in an airstrike in Syria in 2015, her parents became increasingly frustrated with the government’s slow hunt for answers about her death and turned to a former F.B.I. agent for help.
Now, he has unearthed a clue: Ms. Mueller may have been executed on the orders of Abu Bakr al-Baghdadi, the slain Islamic State leader, not in a Jordanian airstrike as the Islamic State long claimed.
ISIS operatives killed Ms. Mueller because she knew the identities of Mr. al-Baghdadi and of Abu Muhammad al-Adnani, the group’s spokesman at the time who was believed to be the second-in-command, and could be a security risk for them, according to Umm Sayyaf, the wife of a close al-Baghdadi associate. She was interviewed by the former F.B.I. agent working with the Mueller family, Ali Soufan.
Ms. Mueller, whom Mr. al-Baghdadi raped repeatedly, was moved from location to location in the final months of her life, Umm Sayyaf said. At one point, Ms. Mueller was moved because the oldest of Mr. al-Baghdadi’s multiple wives assaulted Ms. Mueller, breaking a watch that Mr.-al-Baghdadi had given Ms. Mueller as a Ramadan gift and kicking her out of the home.
The revelations, however dark, provided a glimmer of hope for Ms. Mueller’s family in its long struggle to determine what happened to her and perhaps locate her remains to bring her home. They come weeks after Mr. al-Baghdadi was killed in Syria by American commandos in a raid named in her honor.
“I know that Kayla is most likely gone, and in heaven, but what if she isn’t?” her mother, Marsha Mueller, said in a recent interview. “There was always this 1 percent possibility that — could she possibly be alive? We just need to find her and bring her home, that’s the goal.”
Former officials have said that Mr. al-Baghdadi could have ordered Ms. Mueller’s death because she had become a liability as he sought to avoid capture or death, but the American government has not publicly confirmed Umm Sayyaf’s account.
Earlier this year, Ms. Mueller’s parents partnered with Mr. Soufan, a retired F.B.I. special agent who investigated Al Qaeda and interviewed many terrorists while in the bureau. In late July, he and his colleague interviewed Umm Sayyaf, the nom de guerre of a woman named Nisreen Assad Ibrahim Bahar, the wife of one of Mr. al-Baghdadi’s lieutenants. Ms. Mueller as well as other women and girls enslaved by Mr. al-Baghdadi were imprisoned in her house.
In a lengthy interview conducted in Arabic, Umm Sayyaf told Mr. Soufan that ISIS operatives brought Ms. Mueller to her home in the Syrian town of Shaddadi in September 2014.
During her time there, Ms. Mueller was not told who Mr. al-Baghdadi was, only that he was an important “sheikh,” according to Mr. Soufan’s notes of the interview. Ms. Mueller was told that her new name was “Iman” and she was encouraged to practice Islam. In December 2014, after the escape of several of the Yazidi girls being held in the house, Mr. al-Baghdadi took away Ms. Mueller, said Umm Sayyaf.
In early February 2015, the Islamic State announced that Ms. Mueller had died in an airstrike, publishing a tweet that included photographs of a leveled house and personal details about Ms. Mueller, including her email address and phone number.
At the time, Umm Sayyaf said she and other ISIS families believed the group’s claim. But weeks later, her husband, Abu Sayyaf, told her that Mr. al-Baghdadi and Mr. al-Adnani had Ms. Mueller killed.
“When she inquired with Abu Sayyaf as to why they would kill her, Abu Sayyaf reluctantly said it was because she knew the identity of al-Adnani and Baghdadi, and this was a security issue for them,” Mr. Soufan wrote in a summary of the interview. Umm Sayyaf told Mr. Soufan that she had no reason to believe that her husband was lying.
In 2016, federal prosecutors charged Ms. Sayyaf in the young woman’s death and said she was aware that Mr. al-Baghdadi had raped Ms. Mueller.
The interview also provided additional insights into where Ms. Mueller was taken after she left the home in Shaddadi, the last location that her parents had confirmed. Ms. Mueller was initially taken to Mr. al-Baghdadi’s house in Raqqa, according to Umm Sayyaf, who said that a close friend married to the head of the Islamic State’s media ministry, Abu Hassan, told her. But Mr. al-Baghdadi’s wife kicked her out, apparently in a pique of jealously.
She was then brought to the Hassans’ home, where she spent roughly 10 days before being moved again, this time picked up by Mr. al-Adnani, the ISIS spokesman, said Umm Sayyaf.
Originally from Prescott, Ariz., Ms. Mueller was abducted a day after crossing into Syria on Aug. 4, 2013. She had lived in Turkey for the previous eight months doing humanitarian work.
She went into Syria with a Syrian friend who had been contracted to fix the internet at a Doctors Without Borders compound. They were abducted from their car not long after completing the job.
The militants holding her contacted Ms. Mueller’s family, sending a short video of her and demanding 20 million euros for her release. She was later moved to Raqqa, where she was held with nearly two dozen Western hostages, most of whom were released for ransoms paid by European governments or their employers or relatives.
By the spring of 2014, the Islamic State’s ransom demand for Ms. Mueller had dropped to €5 million, according to her family, about the same as what European governments were paying for their citizens.
That July, President Barack Obama authorized a risky raid at the facility, only to have commandos find that the hostages had been moved. The next month, ISIS beheaded the American journalist James Foley, starting a wave of executions of the American and British captives whose governments appeared to be the only ones that hewed to a strict, no-concessions rule.
Even after the announcement of her death, the Muellers continued to look for her. Ms. Mueller’s mother described herself as tormented by images of her daughter that the Islamic State sent the family, which showed Kayla Mueller lying on her back in a shroud with bruises on her face and a large gash on her right cheek — wounds that experts told them were inconclusive regarding her manner of death, or even whether she was dead at all.
Increasingly frustrated by the Obama administration’s lack of engagement with them and with government officials’ insistence that they were doing everything to save their child — a claim they say was repeatedly undercut by what they later identified as missed opportunities to negotiate her release — the Muellers began their own, private effort to find their daughter, or else her remains, hiring Mr. Soufan.
In early 2018, Kurdish forces captured two British men who participated in jailing Ms. Mueller and abusing Western hostages. The men, Alexanda Kotey and El Shafee Elsheikh, and two other ISIS operatives were known by the hostages as the Beatles because of their accents. They told American interrogators about grave sites, but the military and investigators found no remains for Ms. Mueller or the others at those locations, according to a former senior American intelligence official.
On the morning after the raid that killed Mr. al-Baghdadi, President Trump called the four families of the Americans killed in Syria by ISIS. He promised the Muellers during a conversation of about 20 minutes that he would help find out what happened to Ms. Mueller.
Her mother hopes Mr. Trump can find the answers that have eluded her.
“The more I learn, the more I can be a part of what happened to Kayla,” Ms. Mueller said. “That’s why I need to know everything. I need to be able to walk part of this with her.”
Adam Goldman reported from Washington, and Rukmini Callimachi from New York.