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 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.

Her parents received at least three photos, the New York Times reported, citing two individuals who had been briefed on the family's communication with the Islamic State. Two showed 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 individuals 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 alleged.

The group said on Twitter last week that Mueller died in a building that had been demolished by Jordanian airstrikes, a claim that both the White House and Jordan's government said was unfounded.

Yet the images sent to her family did not completely rule out death in that manner.

One of the two people briefed on the evidence said that Mueller's face did not show puffiness or other concussive effects associated with a bomb blast, making it unlikely that she was killed when the area was hit, as the Islamic State said. But the same person said that she could have been in a nearby building or struck by flying debris.

U.S. officials confirmed that the structure was bombed in coalition airstrikes last week.

The authorities insisted that the building, a weapons storage facility, was a legitimate target and explained that they had conducted detailed surveillance to make sure that no hostages were seen going in or out. But officials had not been able to survey the building around the clock, the Times reported, citing a senior U.S. official who requested anonymity to discuss classified information.

Described by friends and family members as a deeply idealistic young woman eager to help those less fortunate, Mueller was just shy of her 25th birthday on Aug. 4, 2013, when she disappeared in the northern Syrian city of Aleppo.

She had arrived in Syria a day earlier with a Syrian man who has been described as her boyfriend or colleague.

He had been contracted to fix the Internet connection at a Doctors Without Borders office, and employees of the international charity were flabbergasted when Mueller showed up with him.

Syria was then a no-go zone for most international aid workers, said employees of the charity, who explained that they reluctantly housed her overnight and agreed to drive her to a bus station for what was supposed to be her trip back to Turkey.

Her car was ambushed on the way, and she and her Syrian companion were abducted. He was later freed and has declined to speak about what happened.

Once in the hands of the militants, Mueller was forced to wear the hijab and was placed in a cell with female detainees, according to two former hostages held in the same facility.

While many of the male hostages were tortured, the female captives, including three female staffers of Doctors Without Borders, were treated relatively well, according to a European hostage who met Mueller during his monthslong captivity last year. The women were not beaten, he said, and he said he believed that they were not sexually molested.

Mueller was among the hostages sought in a failed U.S. rescue attempt last year, President Barack Obama said.

Obama, in an interview with the website BuzzFeed on Tuesday, confirmed for the first time publicly that Mueller was in the group that included journalist James Foley that U.S. special operations forces tried to rescue in northern Syria, only to discover they had been moved.

The raid was initially disclosed by the White House after Islamic State on Aug. 19 released a video of Foley's beheading.

Obama said the United States will continue to refuse ransom demands from terrorists even though denying them is "as tough as anything I do."

The president spoke twice with Mueller's family, most recently to express his condolences, White House press secretary Josh Earnest said.

"She was an outstanding young woman and had a great spirit," Obama said in the BuzzFeed interview. "I think that spirit will live on."

In Arizona, her extended family and friends gathered by the steps of the Yavapai County courthouse to ponder what drove her to such a dangerous place. They and others described a deeply committed woman who refused to avert her eyes from the suffering of others.

"Kayla has touched the heart of the world," said her aunt Lori Lyon.

Her desire to help solve world problems was already on display in high school, where she became involved with a campaign that aimed to stop Flagstaff, Arizona, city officials from using recycled waste water to make snow on a set of peaks considered sacred to the Hopi people. By the time she enrolled at Northern Arizona University in 2007, the Save the Peaks campaign was just one of an array of causes she was engaged with, says her former classmate, Leslie Alamer, who helped set up a website honoring her friend's legacy

After graduating in 2009, Mueller moved to India, and soon after to Israel. In 2010, she volunteered with the International Solidarity Movement in the Palestinian territories, according to Abdullah Abu Rahma, the group's coordinator in the village of Bil'in.

He said that Mueller joined them in using nonviolent means to protest the Israeli occupation. She lived with families in East Jerusalem in order to try to prevent the demolition of their homes.

Kathleen Day, head of the United Christian ministry at Northern Arizona University, remembers how Mueller used her blog as a way to encourage her peers to get involved. She didn't just write a blog post and leave it at that: She sent it to friends and family, asking them to forward it to others.

"It's not that she's so angelic," Day said. "She saw things and did what she could, whatever she could, however she could."

Information from Bloomberg News was used in this report.