Transcript for ISIS Hostage Kayla Mueller's Courage in Captivity
We begin tonight with the untold story of Kayla Mueller, the young humanitarian aid worker from Arizona who fell into the hands of Isis. A heartbreaking tale from her parents and for the first time from other young women imprisoned with Kayla. They tell us about the brutality she endured and her bravery in the face of the infamous Jihadi John. Here's ABC's Brian Ross. Reporter: The home movies show a young woman in love with life. Traveling the world as a humanitarian aid worker. Making her parents so proud. She just always had such a compassion for everybody around her. You're always learning about yourself through other people, and you're always learning about life. And that process never stops. Reporter: And then one morning at the home of Marcia and Karl Mueller, three years ago this month, came this. My name is Kayla Mueller. I need your help. It's very terrifying here. Reporter: Their 24-year-old daughter Kayla was in the hands of one of the most brutal terror groups the world has ever known. Isis. You just go into almost a catatonic state, I think. You just -- can't even stand up. Reporter: Karl owned an auto repair shop. Marcia a former nurse. It broke my heart but I also saw her strength. She was very strong in what she said. Reporter: Nothing prepared the muellers for what was about to happen. This kind of thing tears people apart. Reporter: Kayla had crossed into Syria at this border checkpoint. Going along with a friend hired to install communications equipment for the humanitarian group doctors without borders. It was August 3rd, 2013. Less than 24 hours after leaving here, Kayla Mueller would be plunged into what can only be described as a hell on Earth. Captured, imprisoned, tortured, raped. No one yet knew or could even imagine just how brutal Isis could be to its hostages. Fear. It's fear of unknown. You don't know what is going to happen. Reporter: As her fellow female hostages describe it to ABC news, Isis was holding Kayla in a 12 foot by 12 foot room with a single lightbulb hanging from the ceiling. There was a little bit of light coming by this small vent. But that was it. It was cold. Dirty. Reporter: After being alone for several months, Kayla was joined by three more women. They gave us black dresses and covered our hair and faces. Reporter: For the first time two Kayla's former cellmates are talking publicly about what happened to Kayla and them at the hands of Isis. We realized that they were actually killers. That they would enjoy killing us. Reporter: The prison they shared with Kayla was located outside the Isis headquarter city of raqqah, Syria. She had a strong faithhat gave her a lot of strength. Reporter: Their Isis guards were led by the british recruit who would later be dubbed Jihadi John. I think it was clear that they hated Americans. She'd scream at her. They would blame her for everything that America has done in the world. Reporter: Back in Arizona, Kayla's parents held out hope that Isis would treat an aid worker gently, that one of the humanitarian groups she was connected to would help negotiate her freedom. But that did not happen. No one would claim her. So they assumed she was a spy. And we found out that her fingernails had been pulled out. Her hair had been shaved. And that she had been tortured. Reporter: And the prospect of a negotiated release grew increasingly unlikely as the muellers found themselves up against the U.S. Policy not to make concessions to terrorists. I firmly hold that the united States government paying ransom to terrorists risks endangering more Americans and funding the very terrorism that we're trying to stop. Reporter: So as hostages from other countries which allow ransom payments were released by Isis, Kayla was left behind. It was a horrible feeling to be released, looking forward to being released, but at the same time leaving someone behind. What'd you say to her? Stay strong. This will end one day. Reporter: Finally, in late may, 2014, Isis made contact with the muellers in this e-mail, likely written by one of the british Isis guards. The conditions of Kayla's returning home safely are no media involvement whatsoever and a cash payment. Reporter: Even better than e-mails, a follow-up with Kayla's actual voice laying out the kidnappers' latest demands, including the release of a Pakistani woman being held in a U.S. Prison. If this is not achievable, they are demanding 5 million euros to ensure my release. Good-bye. Reporter: The muellers say the white house stuck to its policy of no ransom. You asked to meet with president Obama. He refused. We did meet in the white house. With many high-ranking officials. But he did not see us. Reporter: And no one in the white house told them what was about to happen. U.s. Special forces, which had carried out a number of successful high-risk rescue missions like this one, had been authorized to launch over the July fourth weekend a secret operation for Kayla and some 18 other hostages. It turned out that the hostages were not longer at that location. Reporter: One week later, in Arizona. We got a nasty e-mail from Isis that said, because of what your arrogant government attempted to do, your daughter has 30 days to live. If you fail to meet this deadline we'll send you a picture of Kayla's dead body. My name is Karl Mueller, this is my wife Marcia. Reporter: As the 30-day deadline approached the mulers recorded this dramatic personal plea, scripted largely by the FBI and sent to the new Isis leader Al baghdadi. Please, show mercy and use your power to free our daughter. This is a plea for Kayla's life. We are asking for your mercy for Kayla, as we believe you are the only one who can grant us this. Reporter: It seemed to have worked. The deadline for Kayla's execution, on her birthday, August 14th, passed with no word of her fate. A sigh of relief. And a hope that she had seen this part of the video herself. We know god is comforting you and strengthening you. I wish I could see you. Reporter: But it was a short-lived moment of relief. Today the entire world is appalled by the brutal murder of Jim Foley by the terrorist group isil. Reporter: The first of Kayla's fellow American hostages to be murdered. Again, the muellers asked to meet with president Obama. We were told no. Reporter: A few weeks later, another American hostage killed, journalist Steven satloff. Now president Obama agreed to meet with the mulers. The president was I felt very cold. We were told many times, we're doing everything we can, and they weren't. They were not? No, they weren't. Reporter: Later, white house officials would threaten the muellers and other hostage families with criminal prosecution if they tried to pay ransom to gain their loved ones' freedom. I had come to a point where our government is not going to bring Kayla home. Marcia, we need to do something. They are not going to bring Kayla home. Reporter: But as dark and as hopeless as it seemed to the muellers, they did receive one piece of what was described by the FBI as good news. They told us she had been actually taken out of the prison by Abu Bakr Al baghdadi, the khalif of Isis. They believed she was with a family, that she was safe. So my thought was, oh, good, Kayla's able to maybe take care of the children, she's a maid. Reporter: Nothing could have been further from the truth, as the muellers later discovered. The so-called family where their daughter was safe was in fact holding Kayla for the leader of Isis, Al baghdadi. And that's when they said he married her. That's the way they put it. Their way of saying he raped her. Baghdadi chose her. Look at the symbolism in that. He's raping America. My dear, beautiful Kayla, I miss you and love you. Yesterday, Friday, at about 10:00 A.M., we were called -- Good evening, we begin with the breaking headline -- He told us it was out on Twitter. The last American hostage held by Isis -- That you had been killed by a Jordanian air strike. Kayla Mueller, 26 years old, is dead. My eyes are clouding up. Sorry. Still not believing the horrific people had lied so much and were so evil. Reporter: Precisely how Kayla was killed remains a mystery. But the muellers refuse to believe that Kayla was dead until Isis a few days later sent three photos of Kayla's body. I just kept still writing to Kayla. Hey, sweetheart. Have so much to share. You, young lady, have taught me so very much. I will see you soon. Love, mom. Since Kayla's death the Obama administration has promised that it will now allow families to privately raise ransom money without the threat of prosecution. Our thanks to Brian Ross and his team for that report.
This transcript has been automatically generated and may not be 100% accurate.