Kayla Mueller was in Syria for humanitarian work.
But the 25-year-old would eventually prove to be not only a hero to humanity, but a hero to her faith.
ISIS abducted Mueller on August 4, 2013. She was held hostage for 18 months—during which time she was made to appear in a “proof of life” video, saying:
“My name is Kayla Mueller. I need your help. I've been here too long, and I've been very sick, and it's very terrifying here.”
He was also the man that Mueller faced as she defended her Christian faith to the Islamic extremists.
Other hostages have recently told how Jihadi John would parade Mueller in front of them.
Former hostage Daniel Rye told ABC News how Mueller bravely defied her captors:
"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."
Former hostage Patricia Chavez also remembers Mueller's extreme courage and faith:
“She was amazing. She was a really strong girl.”
Swedish hostage Frida Saide told ABC News about how Mueller never allowed her terrifying captors to dampen her spiritual resolve:
“[Mueller had] a strong faith that gave her a lot of strength. As a person, she was a very good friend. She was smart, she was fun to be with. She was very kind, extremely generous.”
Saide goes on to describe Mueller as someone who, beyond being strong, had an exceptional compassion for others, even as her life was threatened:
"She was always considerate of others, even though she herself was in a very difficult situation. She was always concerned for other prisoners.
She never stopped being concerned for the Syrian population living through just horrible things in this war, and still are. She never stopped caring for others."
Eventually, Mueller and several teenage Yazidi girls were handed off to Abu Sayyaf, the oil and gas emir for ISIS, to be used as sex slaves for ISIS “caliph” Abu Bakr al-Baghdadi.
They were raped and tortured almost daily.
But according to one of the Yazidi girls, Mueller’s faith and humanism still could not be shaken.
The girl, who calls herself “Julia,” described to 20/20 how Mueller decided to stay with Sayyaf so that her fellow captives could escape:
"I told Kayla we want to escape and I asked her to come with us.
She told me, 'No, because I am American. If I escape with you, they will do everything to find us again. It is better for you to escape alone. I will stay here.'"
Julia also said Mueller confided that she'd pretended to convert to Islam so that her captors could sexually assault her, which would at least keep her alive for the time being.
In reality, Julia says, Mueller never renounced her faith and remained a Christian in secret—even praying that Julia and the other teenaged girls would “escape and live”:
“I will never forget this sacrifice. She was very good to us. I will never forget.”
According to ISIS, Mueller was killed in a Jordanian airstrike on Syria In February 2015.
The White House denied the claim, and not long after stated that Mueller had died of causes that would not be revealed.