Kayla Mueller, a U.S. hostage taken by Islamic State, suffered unbelievable abuses while being held by her terrorist captors. Through months of physical and sexual abuse, she refused to renounce her Christian faith, even to her death.

Mueller’s story was told by four former hostages who shared cells with her on ABC News’s “20/20” Thursday night. The details they offer are nothing short of harrowing.

[dcquiz] Mueller, a 25-year-old aid worker, spent 18 months in ISIS captivity, including a stint as a sex slave to ISIS’s so-called Caliph Abu Bakr al-Baghdadi. Much of Mueller’s time in captivity was spent in a prison run by infamous British ISIS executioner “Jihadi John” and his fellow British ISIS fighters, referred to as “the Beatles” by the prisoners. Mueller suffered through multiple rapes and various forms of sadistic torture, but despite the hellish existence, her fellow captives say she never lost her spirit.

Fellow hostage Daniel Rye, a Danish photographer, shared an example of Mueller’s bravery when her terrorist captors brought her before some male hostages being held next door to her cell.

“One of the Beatles started to say, ‘Oh this is Kayla and she has been held all by herself. And she is much stronger thank you guys. And she’s much smarter. She converted to Islam.’ And she was like, ‘No, I didn’t,'” said Rye, recounting the story for ABC News in an interview.

“I would not have had the guts to say that. I don’t think so,” he added. “It was very clear that all of us were impressed by the strength that she showed in front of us. That was very clear.”

ISIS attempted to get a ransom from Mueller’s family, forcing her to record a proof-of-life video that was sent to her parents three weeks after she was captured in August, 2013.

Unfortunately, attempts to ransom Mueller would be blocked by the FBI and Obama administration. Additionally, aid organization Doctors Without Borders (MSF) refused to negotiate for Mueller, despite the fact she was captured while helping members of the organization install some equipment in Aleppo, Syria.

“I saw how thin she looked, but I saw her eyes were clear,” Mueller’s mother, Marsha, told ABC News, regarding the first time she saw the ransom video. “She was very steady. So it broke my heart, but I also saw her strength.”

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