Kayla Jean Mueller was a US aid worker from Prescott, Arizona.
Government officials reportedly told CNN that it's unclear how Mueller died.
Mueller's family released this statement (via NBC News): "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."
Here's the full statement from the White House:
