 -- American hostage Kayla Mueller was tortured, verbally abused, forced into slave labor for ISIS commanders in Syria and raped by the group's top leader, but her fellow hostages say she never surrendered hope, she selflessly put the welfare of fellow captives above her own and she even stood up to executioner "Jihadi John" to defend her Christian faith.

Four former hostages who shared cells with Mueller, speaking publicly for the first time about their shared ordeal for ABC News' "20/20" broadcast, "The Girl Left Behind," airing Friday, say the Prescott, Arizona, humanitarian aid worker was a courageous 25-year-old who inspired them.

Their ISIS guards were overseen by the British tough Mohammed Emwazi, who would later be dubbed Jihadi John, as he carried out the beheadings and killings of 10 hostages. The Londoner led three other Britons who oversaw the hostage operation. Their prisoners called them "The Beatles."

In March 2014, Mueller was taken to a room next door several times where male hostages were being held. Former hostages said Emwazi paraded her in front of them to show prisoners about to be released who she was and to offer her own proof-of-life by removing her head scarf and briefly introducing herself.

FULL BRIAN ROSS REPORT: "The Girl Left Behind."

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Former hostage Daniel Rye Ottosen, a Danish freelance photographer, recalled how Mueller turned the tables on the men in black.

"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,'" Ottosen told ABC News.

He admits it surprised him a lot. He had once tried to strangle himself when ISIS guards strung his arms up by chains.

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

The only period in Mueller's 18 horrifying months as an ISIS hostage when she wasn't subjected to some form of torture, verbal abuse, prolonged isolation, sensory deprivation, stress positions, forced labor or sexual assault before she died in captivity was the six weeks she was held at an abandoned oil refinery in Syria, with other Western hostages in 2014.

Because many of the ISIS captives were being individually negotiated for ransom with their governments, employers or families, the women, at least, inside the makeshift prison south of ISIS stronghold Raqqa were not subjected to the kind of abuses that Mueller said she experienced in other ISIS prisons before and after her time there, according to the Mueller family and those held captive with her.

Three of the Westerners released by ISIS and a Yazidi teenager who escaped captivity provided eyewitness accounts to ABC News of Mueller's strength, selflessness and will to survive amid her considerable suffering, including details she gave them of her treatment when she was completely alone for most of her confinement by the terrorist group.

"They would scream at her, and they would, you know, blame her for everything that America has done in the world," Frida Saide, one of three women from Doctors Without Borders who shared a cell with Mueller at the oil refinery, told ABC News in an interview this month.

"They picked her apart," said Patricia Chavez, one of the other Doctors Without Borders aid workers held with Mueller.

In her seventh month of captivity, Mueller's frequent isolation and moves between makeshift prisons in Aleppo and Raqqa was interrupted by the arrival at the oil refinery of Saide, Chavez and a dozen other hostages, including Europeans in the process of being ransomed.

In March and April 2014, the women from Doctors Without Borders and a French journalist carried out three letters Mueller wrote by hand to friends and family, indicating it was finally her turn. The Doctors Without Borders women were made to memorize an ISIS email address, which the hostage takers instructed them to give to her parents.

That eventually led to extraordinary negotiations for her release, the former hostages and the Mueller family said in an ABC News investigation spanning more than two years.

Saide, 35, from Sweden and Chavez, 35, from Peru and Belgium, had not been publicly identified as ISIS hostages before agreeing to speak to ABC News this month about their friend, Mueller. At least six men held with them were eventually executed by the brutal "Beatles," and the experience has left the women traumatized.

"Fear. It's fear of the unknown. You don't know what's going to happen," Chavez recalled of the state of terror in which they lived.

What is now known of Mueller's 18 months of hell in ISIS hands has been drawn from the eyewitness accounts of a handful of fellow Western hostages like Saide and Chavez, who spent those six weeks with her in the spring of 2014 inside the oil refinery, as well as from the Yazidi teen who was imprisoned with Mueller in late 2014.

Towards the end of Mueller's life, after her parents Carl and Marsha Mueller say the FBI and Obama administration had blocked opportunities to help them ransom their daughter, her spirit had apparently dimmed, her parents concluded after recently meeting the Yazidi girl, now 15, for the first time.

Mueller was more optimistic about being freed when she briefly shared a cell with the Doctors Without Borders women in early 2014, the freed hostages said.

When the three women entered Mueller's cell and met her for the first time, they said they had to get over the initial shock of finding out that Mueller was an aid worker taken with her contractor friend and two Doctors Without Borders staffers from inside a Doctors Without Borders vehicle six months earlier. The women said their colleagues had failed to disclose the incident to them before their entry into Syria.

Mueller told them how she had helped her friend, Omar Alkhani, a Doctors Without Borders contractor, install satellite internet at an Aleppo Doctors Without Borders hospital, where they were invited to stay the night, and how they were then were abducted the next day from a Doctors Without Borders vehicle with two staffers as they left for a bus station on Aug. 4, 2013.

Mueller's cellmates in the Raqqa oil refinery — Saide, Chavez and the third Doctors Without Borders woman, whose identity remains confidential — entered Syria in November 2013. They said that in a safety briefing, a Doctors Without Borders official did not tell them that Mueller and three Doctors Without Borders workers had been abducted.

"He said that for Doctors Without Borders, that the risk of kidnapping was not considered very big. It wasn't something that I should worry about," Saide told ABC News. "Kayla had already been abducted from an Doctors Without Borders vehicle only a couple of months before that. But he failed to mention this."

Asked about the omission, Jason Cone, the executive director of Doctors Without Borders in the U.S., told ABC News this week that he wouldn't second-guess decisions by the group's security officers three years ago. He added that Mueller's kidnapping was kept quiet for her security.

"At that time, when they went into Syria, it was the express wishes that they — that this incident not be talked about. That was deemed to be the best possible recourse," he said.

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He also said that the women were not traveling near Aleppo, where Mueller was kidnapped, and therefore their risk assessment was different. Saide and Chavez each dispute that claim, saying they were near Aleppo when they entered Syria from Turkey.

As Mueller's fellow female hostages described it for "20/20," ISIS held the four women in a 12-foot-by-12-foot room of brick whitewashed walls, in what the FBI later called the "pipeline desert prison," with a blacked-out window, a single lightbulb hanging from the ceiling and mattresses and blankets on the floor. They could only tell day from night through a ventilation fan near the ceiling.

"There was a little bit of light coming by this small vent, but that was it," Chavez said.

"It was cold, dirty. We didn't have that much to eat," Saide recalled. "They gave us black dresses and hijab, so to cover our heads and faces."

The women passed their time swapping stories of their families, their boyfriends and describing their respective homelands. They also whiled away the hours drawing, reading the Quran, writing and planning escapes that were all but impossible. Mueller sometimes cracked them up doing impressions of guards, including one brute they called "Edges."

And always in the background were ISIS nasheeds — chanting songs of martyrdom and death — blaring on speakers.

"They played on and on and on," Chavez said.

The relentless nasheeds underscored the real violence of the hostage takers, which the world would see later on Aug. 19, 2014, with the first beheading on video by ISIS of an American captive, journalist James Foley.

Death threats were common and credible since a Russian captive was the first to be shot to death.

"We realized that they were actually killers, that they would enjoy killing us," Saide said.

Mueller told the three Doctors Without Borders women in the cell about her previous six months of confinement, held mostly in isolation except for brief periods when she cared for a 14-year-old Shiite girl and another woman affiliated with Syrian President Bashar al-Assad's regime. All faced various forms of abuse by ISIS.

"She was amazing. She was a really strong girl," Chavez said of Mueller.

For Saide, the younger Mueller at 25 years old had a surprisingly positive personality and "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."

"She was always considerate of others, even though she herself was in a very difficult situation," Saide added. "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."

Sometimes they could hear male prisoners being severely beaten in other rooms, the women said. The Beatles would also take the Doctors Without Borders women to another room alone, shining a bright light in their faces and demanded sensitive personal information.

Saide cannot forgive her former jailers, who, she said, "caused so much pain to me and to others."

Emwazi was killed by a CIA armed drone in Syria last year, which vaporized his car.

And Daniel Rye Ottosen, the Danish freelance photographer, said Mueller and another American hostage, journalist Steven Sotloff, figured out a way to pass letters back and forth — leaving them concealed in the common toilet — creating a makeshift game of Trivial Pursuit.

One day, the Doctors Without Borders women were ordered to appear in a proof-of-life video as the medical aid group negotiated their and two male staffers' release. Mueller was told by ISIS captors to stay out of the picture, the women recalled.

"They used to tell her that nobody cares about her. Like, nobody's going to negotiate for her and, you know, trying to put ideas in her mind that she is different [from] us," Chavez said.

But in March 2014, Mueller was told by ISIS to pen two letters to her family stating the demands for her freedom: the release of convicted al-Qaeda operative Aafia Siddiqui from a U.S. federal prison or 5 million euros. She gave a third letter for her family to the Doctors Without Borders women to smuggle out. Though similarly worded, it included important personal contacts on the back, including her friend and college spiritual adviser, the Rev. Kathleen Day of Flagstaff's Northern Arizona University.

Her first letter was carried out by a French journalist upon his release. And then in March it came time for Saide, Chavez and the third Doctors Without Borders woman to leave, carrying Mueller's other letters.

"She was happy that things were moving for us," Chavez said.

Mueller buried any disappointment that no one had asked her in eight months for proof-of-life questions — an obvious sign of a negotiation effort. However, the Doctors Without Borders women were told by the Beatles to memorize an ISIS email address for the Mueller family to begin negotiations.

"It was a horrible feeling to be released, looking forward to being released but at the same time leaving someone behind," Saide recalled.

Saide and Chavez said they hid encouraging notes in Mueller's blankets and tried to make the best of parting. The women hugged their American friend as tears flowed. Saide told her to "stay strong," that it would end for her soon. But Mueller said nothing.

"I felt that I wouldn't be completely free until she was free," Saide said in her interview with "20/20."

Unbeknownst to Saide and Chavez, they said, Doctors Without Borders' Brussels office, which oversaw Syria operations, withheld the smuggled letter from Carl and Marsha Mueller until mid-April and kept secret the second, ISIS-directed letter — which included a ransom demand — and the ISIS email address until May 22.

Doctors Without Borders officials have attributed the delays to their desire not to interfere with their ongoing negotiations for the release of other staffers still held by ISIS at the time. On Wednesday, the aid group issued a long statement that included the claim that "Kayla herself asked the women not to pass along this other [ISIS-ordered] letter." But Saide told ABC News today that the statement by her former employer was "not true." Chavez agreed, saying, "There was no discussion where Kayla asked us not to pass it on."

An extraordinary negotiation began with 27 emails exchanged between ISIS and the Muellers, whose FBI team composed all of the couple's notes, the family has told ABC News and which U.S. officials have confirmed.

But the U.S. began airstrikes against ISIS positions in Iraq in early August 2014 during the negotiations for Mueller, and the terrorist group soon began beheading on video almost all the remaining Western hostages in stated retaliation soon after.

Once U.S. airstrikes expanded broadly across Iraq and into Syria on Sept. 22, 2014, ISIS stopped responding to the Muellers' negotiation pleas. It is believed that by then Kayla Mueller had been handed over to the oil and gas emir for ISIS, Abu Sayyaf, and his sadistic wife, Umm Sayyaf -- Tunisians who kept the American and a half-dozen Yazidi girls as sex slaves for ISIS "Caliph" Abu Bakr al-Baghdadi.

Among several of the Yazidi girls enslaved alongside Mueller inside the Sayyaf household in the fall of 2014, none was closer to her than a then-13-year-old who has asked to be called "Julia" by ABC News. Yazidi males were subjected to mass murder by ISIS in Iraq, and thousands of Yazidi girls were forced to be sex slaves.

Julia revealed in a "20/20" interview how Mueller — who was frequently raped by al-Baghdadi — passed up a chance at an escape in order to increase the odds for the Yazidi teens, who were able to sneak out of the ISIS kingpin's house late one night in a flight to freedom.

"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,'" Julia said.

"It is better for you to escape alone. I will stay here," Mueller said, according to Julia.

In the Sayyaf household, Mueller went by "Kayla Carl," per the Muslim custom of referring to women with their father's name. All the girls, including Mueller, were beaten by the ISIS family — but Mueller also had to go to al-Baghdadi at night, as ABC News first reported last year.

"Baghdadi took her several times in the night for himself," Julia recounted, noting that Mueller would return later and try to not to cry, though at times she broke down.

She told the girls that part of surviving was being forced to pretend she had converted to Islam so the ISIS leader could sexually assault her, though she still clung secretly to her Christian faith.

"When she was with us, she wanted to encourage us because of also what happened with us," Julia said, noting the girls were taken in the night by ISIS men. "She was very tired every time. She was not crying every night, but she was very tired."

Under a full moon, the Yazidi girls finally made their escape, parting tearfully with their older protective "sister." They eventually made their way back to Irbil, in Kurdistan in northern Iraq, where Julia helped U.S. military intelligence officers find the Sayyaf houses. A Delta Force raid in May of last year resulted in Abu Sayyaf being shot to death and his wife being taken prisoner by the American operators.

Mueller "was praying for us to escape, to survive," said Julia, turning a bracelet on her wrist that she wears to honor Mueller. "I will never forget this sacrifice. She was very good to us. I will never forget."

In February 2015, ISIS claimed Mueller was killed in a Jordanian airstrike in Syria. The White House denied that an airstrike killed her but confirmed her death of unstated causes a few days after the ISIS claim.

At first, Julia refused to believe her friend Mueller could be dead.

Asked her reaction to the announcement on Feb. 6, 2015, Saide said simply, "I was devastated."

ABC News' Rhonda Schwartz, Lee Ferran, Matt McGarry, Engin Bass, Alex Hosenball, Cho Park, Esther Castillejo, Lauren Effron and Paul Shin contributed to this report.