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US hostage Kayla Mueller was killed by Islamic State - not in a Jordanian air strike as the group previously claimed, the Pentagon said.
Asked if there was any doubt who killed the 26-year-old aid worker, Rear Admiral John Kirby replied: "No doubt. ISIL."
He added that intelligence officers were still unsure the exact cause of Ms Mueller's death.
A senior US official earlier told NBC News that IS had sent Mueller's family a photograph of her body.
The childhood best friend of Islamic State hostage Kayla Mueller has made an emotional public statement after the aid worker's death was confirmed by US authorities today.
A tearful Eryn Street said: "I am not yet sure how to live in a world without Kayla, but I do know that we are all living in a better world because of her."
The family of Kayla Mueller were sent a photograph which appeared to show the US aid worker's body, a senior US official has told NBC News.
The official said the photo, which was emailed to the family, appeared "to show her dead with trauma injuries".
Islamic State said last week that the aid worker died in an air strike carried out by Jordanian fighter jets.
But the official expressed doubt on the group's claim, saying the injuries "are not inconsistent with" the sort of injuries that would be suffered by victims of an air strike.
Ms Mueller's injuries could be the result of a number of other causes, he said, adding: "We do not know when or how she died."
John Kerry and Chuck Hagel have expressed sadness at the death of US hostage Kayla Mueller who had been held by Islamic State.
Secretary of State Kerry said: "There are no words to express to Kayla's parents, Marsha and Carl, how sorry we are for their crushing loss, just how much so many wished for and worked to try and secure a better outcome.
"ISIL, and ISIL alone, is the reason Kayla is gone."
Hagel, the US secretary of defence, said: "The world is united in condemning ISIL's continued murder and imprisonment of innocents."
Earlier, President Obama vowed to "find and bring to justice the terrorists who are responsible" for the 26-year-old's death.
David Cameron says he is "deeply saddened" to learn of the death of US hostage Kayla Mueller, claiming her "brutal" Islamic State captors are responsible.
“I am deeply saddened to hear of the death of US aid worker Kayla Mueller and my thoughts are with her friends and family as they deal with this awful news," the prime minister said.
“Let me be clear – those responsible for her death are the terrorists that kidnapped Ms Mueller and who held her against her will ever since.
"Kayla travelled to the region to help men, women and children forced to flee their homes and her death demonstrates yet again the indiscriminate brutality of ISIL.
"We will work with coalition partners to ensure her murderers face justice, however long it takes."
The US will "find and bring to justice" those responsible for Islamic State hostage Kayla Mueller's death, President Barack Obama has vowed.
Confirming the death of the 26-year-old aid worker, Mr Obama said: "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.
"On behalf of the American people, Michelle and I convey our deepest condolences to Kayla’s family – her parents, Marsha and Carl, and her brother Eric and his family – and all of those who loved Kayla dearly.
"Kayla’s compassion and dedication to assisting those in need shows us that even amongst unconscionable evil, the essential decency of humanity can live on."
"ISIL is a hateful and abhorrent terrorist group whose actions stand in stark contrast to the spirit of people like Kayla."
The family of Islamic State hostage Kayla Mueller received a private message from the group with "additional information" which confirmed her death, the White House said.
The family of American IS hostage Kayla Mueller say they are "heartbroken" by news of her death.
“We're heartbroken to share we’ve received confirmation that Kayla Jean Mueller, has lost her life," Carl and Marsha Mueller said in a statement.
"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."
American aid worker Kayla Mueller - who was being held by Islamic State (IS) - has died, President Barack Obama has confirmed.
IS claimed last week that she was killed in an air strike by Jordanian fighter jets.
"It's with profound sadness that we have learned of the death of Kayla Jean Mueller," a White House statement said.
Latest ITV News reports
Following confirmation of the US hostage's death, Kayla Mueller's family have released a copy of a letter she wrote in captivity in 2014.
Here is what we know about American aid worker Kayla Mueller who was being held by Islamic State.