US President Barack Obama and the family of American aid worker Kayla Mueller have confirmed that the last-known US hostage held by Islamic State militants had died, with Obama promising to "find and bring to justice the terrorists who are responsible".
US officials said the family received an email and photograph over the weekend from her captors that confirmed her death, but said the circumstances in which she died remained unclear.
Mueller (26) was captured in August 2013 while leaving a hospital in the northern Syrian city of Aleppo. She previously had been working in Turkey providing humanitarian assistance to refugees from Syria's bloody civil war.
Islamic State said that Mueller was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold of the Islamist militant group.
Jordan and US officials have expressed doubt about the account of her death offered by Islamic State, an al Qaeda offshoot that controls wide areas of Syria and Iraq.
Mueller's family said in a statement on Tuesday that they were "heartbroken" to learn of her death and released a copy of a letter she had written in 2014 while in captivity.
"Over the weekend, the family received a private message from Kayla's ISIL captors containing additional information," White House National Security Council spokeswoman Bernadette Meehan said, using an acronym for Islamic State.
Mueller was Islamic State's only American woman hostage known to have died. The group has beheaded three other Americans, two Britons and two Japanese hostages in recent months.
"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," Obama said in a statement released by the White House.
"ISIL is a hateful and abhorrent terrorist group whose actions stand in stark contrast to the spirit of people like Kayla," Obama added.
Meanwhile, Hackers calling themselves 'CyberCaliphate' threatened the US president and his family when they took control of Newsweek magazine's Twitter account on Tuesday with the words 'Je suIS IS', a reference to Islamic State and the deadly attack at French newspaper Charlie Hebdo.
The group, which also took responsibility for hacking Pentagon social media accounts last month, tweeted "#CyberCaliphate Bloody Valentine's Day #MichelleObama! We're watching you, your girls and your husband!"