U.S. officials and family members of Kayla Mueller, a 26-year-old American aid worker taken captive by the self-proclaimed Islamic State in Syria in 2013, confirmed Tuesday that she has died. But government officials conceded they may never know when and how she perished.

“We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life,” Mueller’s parents 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.”

American officials tell The Daily Beast that there is still no evidence to support the terrorist group’s claims that she was killed in a Jordanian airstrike last week. And Pentagon officials have no plans to investigate those claims.

Despite questions swirling around the circumstances behind her death, U.S. Central Command, which is leading the U.S. effort against ISIS, will not investigate whether Mueller died from a coalition airstrike, as the terror group alleges. With that, there may be no clear understanding of how and when she died. The United States investigates claims of civilian casualties only when they come from “credible sources,” which ISIS is not, Navy Rear Adm. John Kirby, a Pentagon spokesman, told reporters Tuesday afternoon.

“We do not know the circumstances surrounding her death,” Kirby said. “We have no indication that there were civilian casualties as a result of those strikes.”

A government official said Tuesday that U.S. experts have examined a photo that apparently shows Mueller’s body lying on the ground, with a view of her shoulder and face. There are no definitive indications of the cause of death from the photo, and no rubble is visible, the official said. It is also not clear from the photo when she died, the official said.

A Mueller family representative told The Daily Beast that the family had received “a private message from Kayla’s captors over the weekend with information that the intelligence community authenticated and deemed credible. Out of respect for the family, we have no other details to share at this time.”

Separately, a defense official confirmed that “the intelligence community has concluded from the information provided by Kayla’s ISIL captors that Kayla is deceased,” using an alternate acronym for the group. However, the official added, “At this time we are not able to confirm a cause of death” and “we do not have evidence to support ISIL’s claim of civilian casualties at the target struck on Feb. 6 by the Royal Jordanian Air Force, with the support of U.S. military aircrews.”

On Feb. 6, U.S. and Jordanian forces conducted a strike in a rural area 25 miles southwest of Raqqa, hitting roughly 20 buildings that a defense official described as a weapons storage area. Shortly afterward, ISIS distributed a photo of a damaged building, saying it was struck during those attacks and that Mueller died inside. The official said the Pentagon believes that building was among those hit in the strike but does not believe that Mueller was inside. The official said there was no indication before or after the strike that there were any civilians in any of the buildings.

The site had been struck twice before Feb. 6, Kirby said, because sometimes ISIS will return to a struck site believing coalition forces will not return.

Jordan ramped up its campaign against ISIS three days before the strike after the terror group released a 22-minute video showing jihadis burning captured Jordanian pilot Lt. Muadh al Kasasbeh alive while holding him in a cage. Kasasbeh’s F-16 crashed Dec. 24 near Raqqa while he was conducting coalition strikes against the group. The brutality of the pilot’s death led to national outrage across Jordan and promises from the government in Amman to avenge his death.

On Tuesday afternoon, some of Mueller’s family and friends remembered her at a press conference in Arizona. They recalled how Mueller had tried to keep up her spirits while in captivity, even making origami sculptures for her guards. “Kayla had such great empathy… and it was her greatest strength,” said Eryn Street, a family friend of Mueller. Her parents didn’t appear at the press conference.

In a letter from Mueller, written last year while she was imprisoned by ISIS, the young aid worker told her family that she was being treated well and hadn’t been harmed by her captors.“Please know that I am in a safe location, completely unharmed + healthy (put on weight in fact); I have been treated w/utmost respect + kindness.”

She also told her family that she did not want the burden of negotiating her release to fall on them.

Over the weekend, Foreign Policy claimed the Muellers “did not want the U.S. military to launch a risky mission to rescue her,” citing an unnamed military official. The official went on to say that, as a result, “gatekeepers within the U.S. government rejected some military plans” to save Mueller before they were presented to President Obama.

The family tells The Daily Beast that they asked the Obama administration to keep them in the loop about any planned military mission—a request that officials granted, according to a Mueller family spokeswoman. “The family requested to be notified in advance if there were to be a raid, and also just to be kept apprised of their efforts. And the White House agreed,” the spokeswoman said. It was “not true” that the family had asked officials to forgo a rescue mission, she added.

The U.S. military launched a raid in Syria last July to free Mueller and her fellow hostages, but they arrived too late, as ISIS had already moved the prisoners. Strands of Mueller’s hair were reportedly found at a site where ISIS was believed to have been holding her and the other captives.

Mueller had been held in captivity for 18 months.