Kayla Jean Mueller, an American ISIS hostage, has been confirmed dead. Mueller’s family received a private message from her captors over the weekend claiming that Mueller has been killed. This information was authenticated by U.S. officials early last Tuesday.

Though U.S. officials have been unable to confirm the cause of her death, supporters of the Islamic State claimed that she was killed during a Jordanian Airstrike this past Friday. The airstrike was a retaliation against the death of a Jordanian pilot who was captured by ISIS.

Pentagon official Navy Rear Adm. John Kirby commented Tuesday that he was certain ISIS was responsible for Mueller’s death and that is was not a result of the Jordanian Airstrike.

Mueller, an aid worker, has been an ISIS hostage since she was captured leaving a Syrian hospital in August 2013.

“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,” Mueller’s family said in a statement released last Tuesday.

President Barack Obama released a statement claiming, “No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla’s captivity and death.”

Mueller is the fourth American hostage to be killed by ISIS. Her fellow victims include American journalists James Foley and Steven J. Sotloff, as well as Peter Kassig, another American aid worker.

Mueller was held in the same prison in Syria as 22 other ISIS hostages. Most hostages were released in exchange for ransoms, but hostages from the United States and Great Britain, as a result of their no-ransom policies, have either been killed or are still in captivity.

On Wednesday President Barack Obama made a formal ISIS War Powers Request. The request asked for a three-year authorization to use force against ISIS.

“The authorization I propose would provide the flexibility to conduct ground combat operations in other, more limited circumstances, such as rescue operations involving U.S. or coalition personnel or the use of special operations forces to take military action against [ISIS] leadership,” wrote President Obama.

Hearings on the War Powers Request will begin sometime this week.