For one tortured weekend, the parents of Kayla Mueller refused to believe that their daughter was dead. From their home in Prescott, Ariz., they issued an impassioned plea to the Islamic State, which had held her captive since August 2013, and urged the extremist organization to contact them privately with proof of her death. The militants acquiesced and sent at least three photographs of her corpse. 
 
 Those photos are among the few clues about her life and death in captivity, as is a letter that she wrote from her cell last year and that her family made public on Tuesday. 
 
 Two people briefed on the family’s communication with the Islamic State said that her parents had received at least three photos. Two showed Ms. Mueller, who was 26, in a black hijab, or Muslim head covering, that partly obscured her face. Another showed her in a white burial shroud, which is used in traditional Muslim funerals. The images showed bruises on the face, but both people, who reviewed the photographs and asked not to be identified given the sensitivity of the matter, said it remained unclear whether her injuries were consistent with being killed in the rubble of a flattened building, as the Islamic State reported. ||||| [This report was updated Feb. 11 to include a statement from the White House National Security Council.] 
 
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 With the confirmed death of Kayla Mueller, new details have emerged about the American hostage’s experience in ISIS captivity, including the belief by some officials that she had been given over to a commander in the jihadi group. 
 
 "ISIS didn't see her as a hostage or a bargaining chip," a counter-terrorism official told ABC News. 
 
 "Reflections" in the limited collection of intelligence in Syria -- like indirect indications of a person's presence somewhere, such as ISIS fighters referring to her in calls or spotted guarding a sensitive site where she might have been held -- indicated Kayla was still alive and sometimes in the company of an ISIS leader being actively tracked who had custody over her, possibly by forced marriage, officials told ABC News. 
 
 Sometimes she was with her ISIS keeper and other times she was not, two officials said. 
 
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 The U.S. has no ground-level intelligence from Syrian human sources or American spies and so relies on "airborne collection platforms" like satellites and drones, officials have said. 
 
 Mueller's family in a public plea to ISIS on Friday said the terrorists had referred to her as a "guest" in private communications. 
 
 In a letter written by Mueller last year and released by the family today, Mueller said she was "in a safe location, completely unharmed [and] healthy." 
 
 "I have been treated with the utmost respect [and] kindness," she wrote. 
 
 Matt Olsen, former head of the National Counterterrorism Center and current ABC News consultant, said ISIS is known to for the “practice of essentially selling off, or putting young women and girls in the company of ISIS militants." 
 
 A former law enforcement official involved in the American hostage cases told ABC News Mueller was briefly able to communicate with some male hostages by passing them handwritten messages. For the most part, she was kept away from the other hostages, the official said. 
 
 Several counter-terrorism officials told ABC News there were "a few ideas but no real clue" of Mueller’s whereabouts with ISIS in Syria over the months since an August deadline for a $6.6 million ransom with accompanying death threat passed -- but U.S.-led airstrikes in Syria were carefully coordinated to avoid killing hostages. 
 
 "We haven't hit any areas where we thought she might be," another official said. In announcing Mueller’s death, ISIS claimed she had perished in a coalition airstrike launched by Jordan. 
 
 A few weeks ago U.S. officials "lost fidelity" on her status entirely, several officials confirmed. 
 
 The Mueller family had not received a proof of life, likely an audio recording of her voice, since last year, officials and a source close to the hostage case told ABC News. She also was never mentioned or shown in any of the brutal hostage execution videos released by ISIS. 
 
 But recently, evidence of a visual sighting of Kayla last fall in Syria was obtained by intelligence agencies and was deemed credible, several counter-terrorism officials said. But, since the sighting was months ago, it gave no hard indication of her location this year. 
 
 Her last proof of life was in May, and then the ransom demand and death threat deadline passed by in August apparently without her execution. 
 
 Then over the weekend, ISIS provided the family with "information," including photographic evidence that convinced the U.S. intelligence community that Mueller had been killed, according to a senior U.S. official. U.S. officials said it has not been possible to determine when or how Mueller actually died. 
 
 Update: White House National Security Council Statement 
 
 Out of respect for Kayla's family, we have not shared details regarding her time in ISIL [ISIS] captivity. In addition, as we have consistently made clear in the past, we do not discuss intelligence. At this time, given the distress that anonymously-sourced reports regarding Kayla's time in captivity are causing her family, we are providing this comment with the Mueller family's consent. 
 
 Reports that have been published by certain news outlets regarding Kayla's time in ISIL [ISIS] captivity are speculative and unproven at this time. The U.S. Government continues to analyze any information regarding Kayla's captivity but has not corroborated any of the multiple theories regarding the conditions throughout the duration of her captivity. 
 
 For a first-hand account of Kayla's time in captivity, we encourage people to read the letter written by Kayla in captivity, which was released by Kayla's parents earlier this week. ||||| Facebook 
 
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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. ||||| An attempt to rescue Kayla Mueller and other American hostages occurred in a July 4 raid previously disclosed by the Pentagon. 
 
 U.S. special forces commandos conducted a raid in the predawn hours on a prison in Islamic State's Syrian stronghold of Raqqah. 
 
 But the mission was unsuccessful because hostages -- which included Mueller as well as American journalists James Foley and Steven Sotloff -- had already been moved and weren't there at the time. 
 
 "We never stopped trying to get her," said a defense official, who wasn't authorized to speak publicly on the matter. "We never lost that focus." 
 
 President Obama confirmed for the first time Tuesday that Mueller was one of the hostages Delta Force commandos ‎attempted to rescue in a raid on an oil refinery facility in northern Syria in summer 2014. The two dozen commandos arrived a day or two after the hostages had been moved from the site, Obama said. 
 
 "I deployed an entire operation -- at significant risk -- to rescue not only her but the other individuals who had been held, and probably missed them by a day or two," Obama said during an interview with BuzzFeed News. 
 
 Obama said the U.S. has a commitment to devote "enormous" resources to free hostages anywhere in the world.‎ 
 
 Relatives of Mueller, who was abducted while volunteering in Syria, announced Tuesday that they had received confirmation that she is dead. 
 
 CAPTION Friends, family praise Kayla for her empathy, compassion Friends, family praise Kayla for her empathy, compassion CAPTION Friends, family praise Kayla for her empathy, compassion Friends, family praise Kayla for her empathy, compassion CAPTION The family of hostage American aid worker Kayla Mueller says they have received confirmation of her death. The family of hostage American aid worker Kayla Mueller says they have received confirmation of her death. CAPTION White House confirms death of hostage Kayla Mueller White House confirms death of hostage Kayla Mueller 
 
 Islamic State released a video last week claiming that Mueller, 26, was killed in an air attack by coalition forces against the militants in Syria. 
 
 U.S. intelligence officials believe that Mueller is dead but don't accept the militants' account of how she was killed, White House Press Secretary Josh Earnest said in Washington on Tuesday. 
 
 There was no evidence civilians were in the area that was targeted by Jordan on Friday, Earnest said. 
 
 The Pentagon confirmed that Jordanian warplanes bombed a weapons storage facility Friday, but there was no indication that Mueller was there at the time, said Rear Adm. John Kirby, a Pentagon spokesman. 
 
 “We do not know the circumstances surrounding her death,” he said. “U.S. intelligence had no evidence that there were any civilians at the location of this weapons storage facility that was struck by Jordanian F-16s.” 
 
 The building, a facility near the Islamic State stronghold of Raqqah, was hit Friday and at least twice before, he said. 
 
 Regardless of how Mueller died, he said, her Islamic State captors were to blame. “They are responsible for her safety and well-being,” Earnest said. 
 
 Mueller's family members released a statement saying that they had received confirmation of her death. They did not disclose how they learned the information. 
 
 However, a source close to the family who requested anonymity because of the sensitive nature of the communications said a private message containing additional information from the captors was sent to Mueller's parents over the weekend. Once the statement was authenticated by the intelligence community, the family concluded that she was dead, the source said. 
 
 “We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life,” her parents, Carl and Marsha Mueller, and her brother, Eric, said in the statement. 
 
 U.S. officials are aware of at least one other American being held in the region, Earnest said. He did not identify the individual, or the group holding the captive. But he may have been referring to Austin Tice, an American journalist and former Marine who disappeared in Syria in August 2012. 
 
 It is unclear who is holding Tice, but his parents, Debra and Marc Tice, have said that they don't believe he is in the hands of Islamic State militants. 
 
 "Our hearts go out to the Mueller family and everyone who loves Kayla," the family said in a statement posted on a website that aims to raise awareness about Austin Tice's captivity. "We have no way of understanding what they are going through, and we wish them all possible comfort." 
 
 In Washington, the White House released a statement of condolence from President Obama. 
 
 "On behalf of the American people, Michelle and I convey our deepest condolences to Kayla’s family -- her parents, Marsha and Carl, and her brother Eric and his family -- and all of those who loved Kayla dearly," the statement said. "At this time of unimaginable suffering, the country shares in their grief." 
 
 Mueller had a long history of volunteering to help women and children, having worked for aid groups in Arizona before setting out for other countries, including India and Turkey. 
 
 In August 2013, she was abducted in the Syrian city of Aleppo as she left a hospital operated by Doctors Without Borders and was being held in Raqqah, a militant stronghold in northeastern Syria. 
 
 The family statement also included letters Mueller wrote to her family, including one on her father’s birthday in 2011. 
 
 “Some people find God in church. Some people find God in nature. Some people find God in love,” she wrote. “I find God in suffering. I’ve known for some time what my life’s work is, using my hands as tools to relieve suffering.” 
 
 After attending Northern Arizona University, she worked with aid groups in northern India, Israel and the Palestinian territories. She returned to Arizona in 2011, where she worked at an HIV/AIDS clinic and volunteered at a women's shelter. Late that year, she moved to southeastern France and worked as an au pair while learning French in preparation for a planned move to Africa. 
 
 But the plight of families fleeing the violence in war-torn Syria drew her to Turkey in December 2012. She worked with the aid groups Support to Life and the Danish Refugee Council, assisting women and children who crossed into Turkey as refugees. 
 
 She also made some trips into Syria to help reconnect family members separated by the fighting. Her trips into the country took her to Aleppo, where she was eventually kidnapped. 
 
 In correspondence to her family, which she slipped to other detainees who were being freed, Mueller was contrite and seemed to try to assuage her family’s worry. She told them she was being treated well and was not in harm’s way. 
 
 “If you could say I have ‘suffered’ at all throughout this whole experience it is only in knowing how much suffering I have put you all through; I will never ask you to forgive me as I do not deserve forgiveness,” Mueller wrote in a letter the family said it received in the spring of last year. 
 
 She said in the letter that she was willing to wait for her freedom if it meant that her family would be absolved of the burden of negotiating her freedom.
– 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."