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 Friends and family of slain ISIS hostage Kayla Mueller said they hoped her death would achieve what she'd worked for before she was kidnapped: raising the awareness of Syrian refugees. 
 
 "The things that were important to Kayla are finally getting the attention they deserve," aunt Lori Lyon said in a tearful news conference in her hometown of Prescott, Arizona, Tuesday — hours after the family confirmed that Mueller had died while in ISIS' hands. 
 
 The Rev. Kathleen Day, who runs the United Christian Ministry at Northern Arizona University, where Mueller was a student, recalled following her blog posts from Syria. "I hope you can tell the entire world here what I've said here, and what I've seen," Mueller wrote, according to Day. The pastor also recalled Mueller's recollection of refugees' frequent response when they learned she was American: "Where is the world?" 
 
 "We're right here, Syria," Day said. "We've seen your suffering reflected in Kayla's eyes." 
 
 Eryn Street, a close friend of Mueller's, recalled her seemingly limitless empathy. "She had an amazing ability to share in someone else's suffering and add a little happiness to it," Street said. 
 
 She added: "I'm not yet sure how to live in a world without Kayla, but I do know we're living in a better world because of her." 
 
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 In an interview with BuzzFeed News on Tuesday, President Obama called Mueller "an outstanding young woman and a great spirit — and I think that spirit will live on." 
 
 He added, "I think the more people learn about her, the more they appreciate what she stood for and how it stands in contrast with the barbaric organization that held her captive ... but I don't think it's accurate ... to say that the United States government hasn't done everything we could." 
 
 The family of Abdul-Rahman Kassig, another American aid worker who was captured by ISIS and who was murdered by the group, offered its condolences Tuesday. 
 
 We mourn with the Muellers over the death of their daughter, Kayla Mueller. She lived a life of service. She will be remembered! — Kassig Family (@kassigfamily) February 11, 2015 
 
 Mueller, 26, was confirmed dead Tuesday morning, four days after ISIS claimed she'd been killed by a coalition airstrike in Syria. 
 
 A senior U.S. official said the evidence consisted of a photograph emailed to the family and "appears to show her dead with trauma injuries." But the official said the injuries were not consistent with injuries suffered by victims of an airstrike. 
 
 Jen Psaki, a U.S. State Department spokeswoman, said the government were not able to confirm a cause of death. "The U.S. military has indicated that there was no evidence of civilians in the target area prior to any recent coalition airstrikes," Psaki said. 
 
 Kayla Jean Mueller in a photo released by her family. Family Photo 
 
 Pentagon spokesman Rear Admiral John Kirby on Tuesday also dismissed ISIS' assertion that the Jordanian air strike was responsible for Mueller's death. 
 
 "Let's not forget in whose hands this woman died. And let's not forget who's ultimately responsible for it: ISIL," Kirby said, using an alternate acronym for the Islamic State in Iraq and al-Sham. 
 
 Mueller traveled to the Turkish-Syrian border to work with refugees in December 2012, and was taken captive in Aleppo, Syria on Aug. 4, 2013, while leaving a Doctors Without Borders hospital. Her family asked the media to keep her name out of the press out of fears it would endanger her life. 
 
 Mueller was among the U.S. hostages Army Delta Force commandos unsuccessfully tried to rescue in an operation in Raqqa last July. And Congressman Paul Gosar, a Republican whose district encompasses Mueller's hometown, told the Arizona Republic this week that there were other plans to rescue Mueller that either failed or were shot down by her family as too risky. ||||| American special operations forces and FBI agents on the ground in Syria are actively searching sites where they believe Western hostages, including Americans executed by ISIS, may have been buried, counterterrorism officials told ABC News. 
 
 Interested in Syria? Add Syria as an interest to stay up to date on the latest Syria news, video, and analysis from ABC News. Add Interest 
 
 Three officials briefed on the ground searches by U.S. commandos -- the first real effort to recover the remains of two American journalists and two American humanitarian aid workers killed from 2014 to 2015 -- said they were undertaken on the basis of new intelligence from two ISIS members from London captured last month by Syrian Kurds. 
 
 “Intel people are digging hard and directing ground guys to locations,” a counterterrorism official who is not authorized to speak publicly told ABC News. 
 
 Two other counterterrorism officials confirmed to ABC News that ground searches have begun and suggested the efforts are limited so far but could be expanded. The remains are believed to be spread out over several sites. 
 
 Syrian Democratic Forces/Handout via Reuters 
 
 Alexanda Kotey and El Shafee Elsheikh are two of the most important ISIS figures ever captured on the battlefield. U.S. commanders at Joint Special Operations Command and CIA have had a policy of carrying out targeted killings with armed drones of the terrorist group's senior leadership rather than more risky attempts to capture them. 
 
 The two ISIS fighters taken prisoner last month were half of a quartet of British guards — dubbed the "Beatles" by the hostages — who tortured and beheaded their captives. Their leader, Mohammed Emwazi, was called "Jihadi John" and died in a CIA airstrike in Raqqa, Syria, in November 2015. Another member, Aine Davis, was captured in Turkey last year. 
 
 The two men are being interrogated by U.S. officials and are providing information about burial sites of American hostages they tormented and executed while running a kidnap and ransom operation for ISIS, which began in 2013. Ransom demands were halted after a July 4, 2014 failed Delta Force rescue raid and the subsequent U.S.-led coalition air campaign against ISIS. 
 
 AP, FILE 
 
 The U.S. Hostage Recovery Fusion Cell has contacted families of the hostages who died -- some of whom were shown having their heads cut off by Jihadi John on video -- many of whom welcomed the capture of the pair and U.S. efforts to find the remains of their loved ones. 
 
 Diane Foley, mother of journalist James Foley, the first to die on video in August 2014, told ABC News, that knowing that U.S. forces are searching for burial sites of her son and his fellow captives is emotionally difficult but gratifying. 
 
 "I am grateful for all these efforts but bringing these two jihadists to justice gives us more hope and solace than the return of Jim's remains," she said. 
 
 Foley's execution was followed by the killings on video of fellow American hostages Steven Sotloff, a journalist, and Abdul-Rahman (Peter) Kassig, a humanitarian medic and former Army Ranger. Five more men — two from Britain, two from Japan and one from Jordan — also died in horrific videos released over several weeks by ISIS into early 2015. 
 
 Kayla Mueller was the only westerner whose death did not play out in a Jihadi John video. ISIS announced the death of the humanitarian aid worker from Prescott, Ariz. in a Feb. 6, 2015 tweet, claiming a Jordanian airstrike killed her in Syria. The captors sent her parents three photos of her lifeless face when they asked for proof. 
 
 "I don't know what happened to Kayla. I don't where Kayla is. I need her home," said her mother, Marsha, on Friday. "She belongs here." 
 
 Matt Hinshaw/The Daily Courier via AP, FILE 
 
 Kayla's father, Carl Mueller, said he appreciates that any effort to recover the remains of his only daughter, whose death is still a mystery. The Muellers recently pressed U.S. hostage recovery officials in a Washington briefing to reveal how their daughter died and were told specific details have eluded investigators. 
 
 "Unlike the other families, we don't have the gruesome execution video,” Carl Mueller told ABC News. “We need to know she's gone.” 
 
 Beyond the effort to examine sites in Syria, the Muellers want any information about Kayla's last days, when according to fellow captives who later escaped she was forced to be the sex slave of Abu Bakr al-Baghdadi, the leader of ISIS. Sources have told ABC News that al-Baghdadi is believed to have been most likely killed in a U.S. airstrike in July, though his death was never confirmed. 
 
 "I would like anyone who has information about Kayla to share that with us," her mother said. 
 
 That includes Kotey and Elsheikh, who may know how she died and where she is buried. 
 
 According to J.M. Berger, author of “ISIS: The State of Terror,” Kotey and Elsheikh are the most important ISIS figures captured in several years. 
 
 "This is a really big development," Berger said. "ISIS has provided an incredible documentation of its atrocities and we haven’t seen anyone held to account for that. It would be hard to see how these guys could get off. People need to see justice being done." ||||| Too Slow 
 
 The U.S. did not think British intelligence was good enough to act on and waited a month to launch a mission to rescue ISIS's hostages. By then it was too late. 
 
 The U.S. government obtained intelligence on the possible location of American captives held by ISIS in Syria last year, but Obama administration officials waited nearly a month to launch a rescue mission because of concerns that the intelligence wasn’t conclusive and some of it had come from a foreign service, U.S. and British officials told The Daily Beast. 
 
 British officials, as well as private security contractors, said they were frustrated by Washington’s hesitance to give the go-ahead for a rescue attempt, which eventually was carried out on July 4, 2014, by which time the hostages had been moved. The following month, ISIS began beheading its American and British prisoners in a series of grisly Internet videos. 
 
 Toward the end of May, the British government had identified two or three locations in and around the Syrian city of Raqqa, the de facto capital of the so-called Islamic State, where the militants had moved hostages during the previous weeks and months. But the British were not absolutely sure in which location the Westerners were held. The captives included American journalists James Foley and Steven Sotloff, as well aid worker Kayla Mueller. The information—based on debriefings of European captives who had been released, satellite and drone surveillance, and electronic eavesdropping—was not definitive in May. 
 
 Then, in early June, London had a “positive identification and that information was shared with Washington,” said a British source. The delay of nearly a month before the rescue bid was mounted remains a source of bewilderment for British officials. 
 
 But a U.S. official said that inside the White House, Obama’s senior national-security advisers were not willing to base a raid on intelligence developed by a foreign service. “The issue was that they didn’t trust it, and they wanted to develop and mature the intelligence, because it wasn’t our own,” said the U.S. official, who asked to remain anonymous when discussing sensitive hostage-rescue efforts. 
 
 “They got the information. They just didn’t trust it. And they did sit on it, there’s no doubt about that,” the official said. 
 
 Bernadette Meehan, National Security Council spokesperson, told The Daily Beast: “U.S. forces conducted this [rescue] operation as soon as the president and his national-security team were confident the mission could be carried out successfully and consistent with our policies for undertaking such operations.” 
 
 But Diane Foley, the mother of James Foley, who was the first American to be shown murdered on camera, also raised questions about the timing of the rescue effort, telling The Daily Beast that French officials had developed information about the hostages’ location as early as March, but that the U.S. government didn’t act on it. 
 
 “That was part of our frustration,” she said. “The State Department said they were connecting with the French and everybody at the highest levels.” And yet, there was no movement on the U.S. side. “Very specific information was available as early as mid-March. And that’s what’s been so tough for us as families, because apparently they were held in the same place all those months,” Foley said. 
 
 Last August, after the administration disclosed the failed raid, National Security Adviser Ben Rhodes pushed back against the suggestion that the United States hadn’t done enough to win the release of the American captives. 
 
 “I can assure you that we have done everything that we can possibly do to try to bring home our hostages,” Rhodes said during a press briefing on Martha’s Vineyard. “It’s an incredibly difficult circumstance in a place like Syria, again, where you have such a violent conflict raging. But we’ve used all of our military, intelligence, diplomatic resources that we can bring to bear to try to pull a thread to find out where our hostages are; to try to rescue them when we saw an opportunity; to try to work with any country that might have any means of locating them.” 
 
 U.S. intelligence agencies had also debriefed European hostages who were released after their governments made ransom payments. The Washington Post reported last August that the president had authorized a rescue attempt “earlier this summer,” citing a senior administration official. 
 
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 “We had a combination of... intelligence that was sufficient to enable us to act on it,” the official told the Post, and the military moved “very aggressively, very quickly to try and recover our citizens.” 
 
 Rescue missions are inherently dangerous, and recent attempts have ended in the death of the hostages. But family members of the Americans held by ISIS, including the parents of Foley and Sotloff, as well as Foley’s brother, have said the U.S. government did not do enough to rescue their loved ones. 
 
 Responding to the general criticism of the United States being slow to act, President Obama told BuzzFeed this week, “I don’t think it’s accurate then to say that the United States government hasn’t done everything that we could. We devoted enormous resources—and always devote enormous resources—to freeing captives or hostages anywhere in the world, and I deployed an entire operation at significant risk to rescue not only [Mueller], but the other individuals that had been held, and probably missed them by a day or two, precisely because we had that commitment.” 
 
 The question of U.S. rescue efforts has taken center stage again with the death of Mueller, a 26-year-old aid worker who was taken captive in Syria in 2013. U.S. officials and Mueller’s family confirmed this week that she was killed while held hostage by ISIS, although the cause of her death has not been determined. 
 
 Questions are now being asked of Mueller’s then-boyfriend, who was with her when they and two others were snatched by ISIS after visiting a Doctors Without Borders hospital near Aleppo on Aug. 4, 2013. He is a Damascus-born freelance photographer who had also undertaken some projects for the Danish Refugee Council and worked for a time as a staff reporter at Reuters. The pair met in Cairo, according to a former colleague who worked with the man. 
 
 After the pair were detained, ISIS let Mueller’s boyfriend go—a move that later prompted some speculation that he might have been involved in the abduction. But those fears were dispelled when, at great risk to himself, he went to Raqqa to try to negotiate her freedom. He claimed to be her husband, but Mueller reportedly told her captives that she wasn’t married. It’s not clear whether she was aware that her boyfriend had returned for her. But he was seized and held for two months, from December 2013 to January 2014, before being set free again. 
 
 “He was besotted with [Kayla],” said the man’s former colleague. “There was talk before her capture of an engagement.” In an email to The Daily Beast, the photographer declined to be interviewed, writing, “let me take my time to understand what happened and to bring myself all together.” He has also declined interview requests from private security contractors working on hostage cases. 
 
 Writing under the pseudonym Omar Alkhani on Facebook, the man has posted romantic missives to Mueller. In a post apparently written after learning of confirmation of Mueller’s death, he bid her farewell. “Thank you for coming into my life and giving me joy, thank you for loving me and receiving my love in return,” he wrote in one post. “You were everything I wanted. You were so beautiful and charming, and you supported me in everything I did, even if it was extremely stupid.” 
 
 “Alkhani” generally avoided writing about her after her capture, but in September 2013 posted, “My heart is in your detention cell/it lives your pain all day/you are not alone/you are not alone.” 
 
 On Wednesday, reports surfaced that Mueller may have been given as a bride to an ISIS fighter. But British sources questioned the claim, saying it was the first they had heard of it and it didn’t crop up in any “chatter they were monitoring” or in communications between London and Washington. “I find the report dubious: She had potential monetary value for them as far as they were concerned,” says a British source. 
 
 ISIS claims that Mueller was killed last week in a Jordanian airstrike conducted in revenge for the group burning alive a Jordanian pilot. The Jordanians dismissed the allegation as ISIS “spin,” and U.S. officials have said they have no information on how Mueller died. ||||| 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. 
 
 Interested in ISIS? Add ISIS as an interest to stay up to date on the latest ISIS news, video, and analysis from ABC News. Add Interest 
 
 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." 
 
 Play 
 
 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." 
 
 Daniel Rye Ottosen 
 
 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. 
 
 ABC News 
 
 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. 
 
 Play 
 
 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. 
 
 AP Photo 
 
 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.
– President Obama says his reaction to the death of ISIS hostage Kayla Mueller is "heartbreak"—but he thinks her "great spirit" will live on. "The more people learn about her, the more they appreciate what she stood for—and how it stands in contrast with the barbaric organization that held her captive," he tells BuzzFeed in an interview yesterday. He stresses the government did all it could to free her, including deploying an entire operation "at significant risk" to free her and other US hostages. The government did, however, stick to its policy of not paying ransoms to organizations like ISIS; ABC News reports ISIS demanded $6.6 million for her release, and had set an August 2014 deadline for payment. More emerging details: Although the government says it has been unable to confirm the cause of Mueller's death, per the New York Times, ISIS sent Mueller's family three photos of her body, according to two people briefed on the family's correspondence with ISIS. Two photos reportedly showed Mueller in a black Muslim hijab that partly covered her face; the third showed her in a traditional white burial shroud. And while the Times sources said they saw bruises on Mueller's face in the photos, there was no evidence of "puffiness or other concussive effects associated with a bomb blast," though they acknowledge she could have been located in a nearby building or injured by flying debris. The rescue operation referenced by Obama took place July 4 in a predawn raid on a Raqqa prison by Delta Force commandos, the Los Angeles Times reports. But the two dozen soldiers were too late: The hostages—who also included James Foley and Steven Sotloff—had apparently been moved a day or two before. "We never stopped trying to get her," a defense official not authorized to speak publicly tells the Times. "We never lost that focus." Officials yesterday told ABC News that Mueller may have been "given over" to an ISIS commander, and that forced marriage could have even been involved. "ISIS didn't see her as a hostage or a bargaining chip," said one. Devastated friends and relatives in Mueller's hometown of Prescott, Ariz., say they hope her death will raise awareness of the plight of the Syrian refugees she worked with, NBC reports. "The things that were important to Kayla are finally getting the attention they deserve," an aunt said. "The world grieves with us."