U.S. officials and family members have confirmed an American woman held hostage in Syria is dead, four days after Islamic State militants said a coalition air raid killed the aid worker.
White House Spokesman Josh Earnest said U.S. intelligence analysts have not determined when or how 26-year-old Kayla Mueller died after being kidnapped in August 2013.
Claims surfaced online late last week that a Jordanian airstrike hit a building where Mueller was believed to be held in the Syrian city of Raqqa. Jordanian, U.S. and other officials raised questions about the claim.
Earnest said a Jordanian airstrike on February 6 was conducted in coordination with the United States near Raqqa, but "there is no evidence of civilians in the target area.... That certainly would call into question the claims that are made by ISIL."
"What is not possible to call into question is that ISIL, regardless of her cause of death, is responsible for it," he said. "This, after all, is the organization that was holding her against her will. That means they are responsible for her safety and for her well-being. And they are, therefore, responsible for her death."
National Security Council spokeswoman Bernadette Meehan said Mueller's family received a private message from Islamic State captors over the weekend containing "additional information."
“Once this information was authenticated by the intelligence community, they concluded that Kayla was deceased," Meehan said.
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U.S. Secretary of State John Kerry said the Islamic State alone "is the reason Kayla is gone."
"Like our friends in Jordan, our resolve is unshaken to defeat this vile and unspeakably ugly insult to the civilized world and to defeat terrorists whose actions – killing women, killing children, burning people alive – are an insult to the religion they falsely claim to represent," he said.
In a written statement, President Barack Obama said, "She has been taken from us, but her legacy endures, inspiring all those who fight, each in their own way, for what is just and what is decent. 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."
U.S. Senator John McCain, who represents Mueller's home state of Arizona, said Tuesday that "Kayla’s remarkable legacy of service will never be forgotten, even by so many who never had the honor of meeting her."
Mueller's parents were "heartbroken" over their daughter's death, they said in a statement.
"We are heartbroken to share that we've received confirmation that Kayla Jean Mueller has lost her life," Carl and Marsha Mueller said in the 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."
In a letter made public by the family, the aid worker wrote during her captivity that she was being treated with the "utmost respect [and] kindness" at a "safe location, completely unharmed [and] healthy."
"If you could say I have 'suffered' at all throughout this whole experience, it is only in knowing how much suffering I have put you all through," Mueller wrote on a notebook page. "I have surrendered myself to our Creator."
"None of us could have known it would be this long, but know I am also fighting from my side in the ways I am able. I have a lot of fight left inside of me. I am not breaking down [and] I will not give in no matter how long it takes," she added.
Aru Pande contributed to this report from the White House, Jeff Seldin contributed from Washington. Some additional material was provided by Reuters.