U.S. President Barack Obama vowed to bring to justice those responsible for the abduction of American aid worker Kayla Mueller, whom the White House and her family confirmed dead on Tuesday.

Mueller, 26, is the last-known U.S. hostage held by Islamic State militants.

Security officials said the aid worker's family received an email and picture over the weekend from her kidnappers that showed her dead but how she died remained uncertain.

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Mueller was taken hostage in August 2013 while on her way out of a hospital in Syria's northern city of Aleppo. She used to work in Turkey assisting refugees from war-torn Syria.

On Friday, Islamic State claimed Mueller lost her life when Jordanian fighter jets blew up a building where she was kept outside the group's stronghold city of Raqqa.

Jordan and U.S. officials doubt the Islamic State's account of Mueller's death. Her family said on Tuesday they were "heartbroken" that her death had been confirmed.

White House National Security Council spokeswoman Bernadette Meehan said a private message from Mueller's ISIL captors containing more information had been sent to her family," using an acronym for Islamic State.

Meehan said once the intelligence community verified the information, they concluded that Mueller was indeed deceased.

The aid worker was Islamic State's only hostage known not to have died by decapitation. The militants had beheaded two Japanese, two British and three other Americans -- most of them journalists or aid workers -- in recent weeks.

In a statement released by the White House, Obama pledged that no matter how long it takes, the U.S. will find the "terrorists" who are behind Mueller's death and captivity. Last year, he ordered U.S.-led air strikes against Islamic State positions.

Obama added that ISIL is an abhorrent and hateful terrorist group whose actions stand in stark contrast to the spirit of people like Mueller.

TagsKayla Mueller, ISIS, ISIL, Hostage, U.S. airstrike