Kayla Mueller Defined Strength & Courage While Being Held By ISIS

In 2013, 24-year-old American foreign aid worker Kayla Mueller was captured by ISIS while volunteering with Doctors Without Borders in the war-torn city of Aleppo, Syria. What followed was a harrowing captivity that lasted more than a year, before she was ultimately killed in 2015. She was one of dozens of Americans who have been abducted by the militant Islamic group, and according to some former hostages who met her while in prison, she left a profoundly courageous impression. In short, Mueller defined strength while captured by ISIS, at one point refusing to renounce her Christian faith despite the insistence of her captors.

Specifically, according to a report set to be aired on ABC's 20/20 on Aug. 26, four of Mueller's former cellmates told the story of how she stood up to her captors, despite the horrifying treatment she'd received. She was reportedly the victim of rape, torture, and forced manual labor, ABC News reported, a particular target of the militants as an American woman and a Christian.

ABC News teased the segment on Thursday, Aug. 25, including Danish photographer Daniel Rye Ottosen who described Mueller defying one of the most infamous members of ISIS' forward-facing operations, the Kuwaiti-born Briton Mohammed Emwazi, who'd ultimately become widely known by the media-bestowed moniker "Jihadi John."

According to Ottosen, one of Emwazi's cohorts ― part of the quartet of British ISIS guards the prisoners referred to as "the Beatles" ― tried to tell the other captives that Mueller had renounced her Christianity and joined Islam, but even under the most duress imaginable, she didn't go along with it:

One of the Beatles started to say, 'Oh, this is Kayla, and she has been held all by herself. And she is much stronger than you guys. And she's much smarter. She converted to Islam.' And then she was like, 'No, I didn't.' ... I would not have had the guts to say that. I don't think so. It was very clear that all of us were impressed by the strength that she showed in front of us. That was very clear.

According to ABC News, one of the other women captured while working for Doctors Without Borders testified to Mueller's resilience in the face of her harrowing situation, as well, describing how she was treated while being held in an oil refinery in Raqqa, Syria. The oil refinery was reportedly used to house hostages pending negotiations with their families or their government ― negotiations which the United States, in a long-established standard, does not participate in with terrorist organizations.

According to former ISIS prisoner Frita Saide, who was also working with Doctors Without Borders when she was abducted by the militant group, while Mueller wasn't subject to the same sexual and physical abuse she was while outside the refinery, she still had to show resilience in the face of sustained psychological torment: "They would scream at her, and they would, you know, blame her for everything that America has done in the world," ABC News reported.

If you're curious to learn more about Mueller's story and the legacy she leaves behind, you'll definitely want to check out 20/20's report on Aug. 26 at 10 p.m. ET. It'll also include some newly released footage of a previously unseen proof-of-life video of Mueller shot back in 2013. Needless to say, it's quite disturbing, so you should exercise your best judgment on whether or not to watch. But if you do want to watch it, you can see some of it here.