The Pentagon said Tuesday that Kayla Jean Mueller, an American held hostage by the Islamic State group, was killed by ISIS militants and not by a Jordanian airstrike, the Associated Press reported. Rear Adm. John Kirby told AP it’s unclear exactly how Mueller died but there’s “no doubt” ISIS killed the 26-year-old aid worker.

Mueller’s parents, Carl and Marsha, confirmed her death earlier Tuesday, but did not elaborate on the manner of her death or how they received confirmation. “We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life. 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,” the Mueller family of Prescott, Arizona, said in a statement, according to NBC News.

The Islamic State, also known as ISIS or ISIL, said Mueller was killed last Wednesday in a Jordanian airstrike aimed at the group’s de facto capital city of Raqqa in northern Syria. Jordan launched airstrikes against the militants in Iraq and Syria one day after the group released a video showing a Jordanian pilot being burned alive. U.S. officials initially questioned the claim last week that Mueller had been killed in an airstrike. “We have not at this time seen any evidence that corroborates [ISIS’ statement],” the White House said in a press release Friday.

Mueller was captured in August 2013 as she left a hospital in northern Syria. She was the only known remaining American prisoner of the Sunni militant group, the Associated Press reported.