(CNN) -- Kayla Mueller's parents had been holding out hope.
But on Tuesday, the family of the American being held hostage by ISIS revealed devastating news.
They received it, officials said, in a message from her captors.
"We are heartbroken to share that we've received confirmation that Kayla Jean Mueller has lost her life," the family said in a statement. "Kayla was a compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice and peace."
ISIS sent the family a private message over the weekend, National Security Council spokeswoman Bernadette Meehan said.
"Once this information was authenticated by the intelligence community, they concluded that Kayla was deceased," Meehan said.
The message sent to the family included photos. One picture showed her wrapped in a burial shroud, but there was enough showing for the family and forensics examiners to identify her, a U.S. official briefed on the matter told CNN.
The photos also showed bruises on the face, The New York Times reported, but it was unclear whether her injuries were consistent with being killed in the rubble of a flattened building, as ISIS claims.
The new information does not confirm how Mueller died, a law enforcement source familiar with the case said on condition of anonymity.
On Friday, ISIS said that Mueller, 26, an aid worker captured in northern Syria in 2013, had been killed in a building hit during a Jordanian airstrike on Raqqa, the militants' de facto capital in Syria. At the time, ISIS offered no proof to back up its claim, other than an image of a building in rubble.
But a White House spokesman on Tuesday placed blame for her death squarely on ISIS.
"This was, after all, the organization that was holding her against her will. That means they were responsible for her safety and well-being, and they are therefore responsible for her death," spokesman Josh Earnest said.
Confirmation of Mueller's death drew condolences and tributes from across the country and around the world. In Jordan, where seething leaders have vowed revenge after ISIS burned a captive Jordanian pilot to death, government spokesman Mohammed Al-Momani expressed "grief and anger" over Mueller's death.
Arizona Gov. Doug Ducey said he had ordered flags at state government buildings to fly at half-staff until sundown Wednesday in her honor.
Speaking in Mueller's hometown of Prescott, Arizona, family and friends said they were still finding strength in her seemingly boundless desire to help those in need and share their stories.
Kathleen Day, a friend of Mueller, read from a blog post the aid worker wrote in Syria before her capture: "Every human being should act. They should stop this violence. People are fleeing. We can't bear this. It's too much. I hope you can tell the entire world what I have said here, and what I've seen."
That, Day said, is what friends and family will do now.
"They tried to silence her. They locked her up. They kept us silent out of fear. But now she's free, and she says that she found freedom even in captivity, and that she is grateful, so her light shines," Day told reporters. "And we thank you for shining your light not on Kayla, but shine your light on the suffering that Kayla saw. And let's tell Syria, we hear you, and we're going to do something."
Rescue attempts failed
Mueller made it her life's work to help others. She graduated from Northern Arizona University in 2009 and worked with humanitarian groups in northern India, Israel and Palestinian territories, a family spokeswoman said.
"She had a quiet, calming presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice. ... Kayla's calling was to help those who were suffering, whether in her home in Prescott, or on the other side of the world," her aunts, Lori Lyon and Terri Crippes, said Tuesday.
In August 2013, Mueller fell into the hands of hostage-takers in Aleppo, Syria, her family said, after leaving a Doctors Without Borders hospital.
Her family said ISIS contacted them in May with proof that she was alive. The militants eventually said they would kill her if the family didn't pay nearly $7 million by August 13, according to a source close to the family. What happened after that deadline is unclear.
A number of rescue and negotiation attempts to free Mueller failed, officials said.
Rep. Paul Gosar, R-Arizona, told CNN's Jake Tapper that he and his staffers tried to facilitate Mueller's release on several occasions. Negotiations between the family and ISIS at one point, he said, included discussion about whether Mueller could be swapped for a prisoner being held in Texas who was convicted for conspiring with the enemy.
U.S. President Barack Obama said Tuesday that the government had worked to free Mueller and other hostages.
"We devoted enormous resources, 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 her but the other individuals who had been held, and probably missed them by a day or two, precisely because we had that commitment," Obama said in an interview with BuzzFeed News.
Obama has ordered a review of the U.S. hostage policy, which has been to never negotiate with terrorists. But the President noted Tuesday that the United States has stuck to its policy of not paying ransoms to terrorists.
"The reason is that once we start doing that, not only are we financing their slaughter of innocent people and strengthening their organization, but we're actually making Americans even greater targets for future kidnappings," he said. "So it's as tough as anything that I do -- having conversations with parents who understandably want, by any means necessary, for their children to be safe. We will do everything we can short of providing an incentive for future Americans to be caught."
Obama called the Mueller family Monday night, a U.S. official said.
"He committed that we will relentlessly pursue the terrorists responsible for Kayla's captivity and death," said Meehan, the National Security Council spokeswoman, "and underscored that his team stands ready to help the family in the difficult weeks and months ahead."
Letter gives glimpse into time in captivity
In a letter released Tuesday that Mueller's family said she wrote while in captivity last spring, the hostage entreated her family not to become consumed by efforts to free her.
"I DO NOT want the negotiations for my release to be your duty, if there is any other option take it, even if it takes more time," the letter said. "This should never have become your burden."
Parts of the letter sound despondent, describing how much she misses her family.
Mueller said she could write the letter only a paragraph at a time. "Just the thought of you sends me into fits of tears," the letter says, and "all in all in the end the only one you really have is God."
But she also wrote that she was fighting to survive.
"I am not breaking down + I will not give in no matter how long it takes," the letter says.
"Please know that I am in a safe location, completely unharmed + healthy (put on weight in fact); I have been treated w/the utmost respect + kindness."
The message, Day said, showed that even in prison, Mueller continued to be free.
Others held captive with her told her family and friends that Mueller stood on her head as prisoners tried to exercise in a small space. And she tried to teach the guards to make crafts, showing them how to make origami peace cranes.
"We just delight in that," Day said, "knowing that Kayla remained Kayla."