Previously unknown information is being uncovered about Kayla Mueller’s 18 months in captivity on 20/20The Girl Left Behind.”

Mueller, a 25-year-old humanitarian and activist from Arizona, was kidnapped by ISIS in August 2013 in Syria.

Information about Mueller’s time is being pieced together more than a year after her death through interviews with others who were held as hostages alongside her throughout her lengthy time in captivity.

Mueller is said to have been tortured, abused, forced into slave labor and raped, but she continued to be hopeful and stood up for her Christian faith.

The special also includes a proof of life video that was made in 2013, shortly after Mueller had been taken hostage.

The video was only 10 seconds long and featured Mueller in a black headscarf begging for help.

“I need your help. I’ve been here too long and I’ve been very sick and it’s very terrifying here.”

One of her fellow hostages, Daniel Rye Ottosen, a Danish photograph, recounts the story of how Mueller remained a firm Christian.

He told ABC News:

One of the Beatles (a name the hostages gave their guards) 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.’

The special also interviews a young Yazidi girl who shares the story of how Mueller gave up a chance to escape with other young hostages who were held as sex slaves so they would have an easier time getting away.

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.

The special will air tonight.