Kayla Mueller, a US aid worker held hostage by the Islamic State extremist militia in Syria, is dead, the White House and her family confirmed on Wednesday.
The IS group announced last week that Mueller, 26, was killed on Friday in a Jordanian airstrike.
However, the claim that the airstrike killed Mueller has not been confirmed and Jordan has dismissed the claim.
In a related development, US President Barack Obama said in an interview that Mueller had been among the IS hostages in Syria that the US tried to rescue last summer.
The White House said it had no conclusive information about when or how Mueller died, but that intelligence had confirmed the death based on information provided to the family.
White House spokesman, Josh Earnest, said the target hit by the Jordanians was a weapons compound and there was no evidence of civilians in the area, calling into question IS’s claims about the cause of Mueller’s death.
Earnest said: “What is not possible to call into question is that ISIL, regardless of her cause of death, is responsible for it.
“This, after all, is the organisation that was holding her against her will.
“That means they are responsible for her safety and her well being, and they are, therefore, responsible for her death.”
Mueller, held captive since August 2013, was the last known US hostage in the custody of the extremist group, which has executed foreign journalists and aid workers, in addition to Syrians.
“At least one other American however is being held hostage in the region,” Earnest confirmed.
The family of US journalist Austin Tice has been conducting a public campaign to secure his release after he was kidnapped in Syria, but it is unclear which group might be holding him.
“Our hearts are breaking for our only daughter,” the Mueller family said in a statement, adding that they had been told she was killed.
Obama, who called IS a “hateful and abhorrent terrorist group”, pledged to bring to justice those responsible for her captivity and death.
“He called the Mueller family twice in recent days,” Earnest said.
It was revealed following the beheading in August 2014 of US journalist James Foley that the US military had tried unsuccessfully to rescue a number of hostages, but found that the hostages had been moved before the raid.
Report says US refuses to pay ransom for American hostages, maintaining that paying ransom only encourages kidnappings of Americans abroad and would also fund terrorist groups.
According to an investigation by The New York Times last year, European nations have paid over $100 million to terrorists
But no reconsideration is to be given to the no-ransom policy.
“Kayla dedicated her life to helping others in need at home and around the world,” Obama said in a statement.
He said her compassion and dedication to helping those in need “shows us that even amongst unconscionable evil, the essential decency of humanity can live on”.
dpa/NAN.