President Barack Obama has paid tribute to American aid worker Kayla Mueller, who was killed while being held by Islamic State militants in Syria.
"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," President Obama said in written statement. He vowed the U.S. will "find and bring to justice the terrorists" responsible for her captivity and death.
Officials and family members confirmed Mueller's death Tuesday, four days after Islamic State militants said a coalition air raid in Raqqa, Syria killed the 26-year old aid worker.
White House spokesman Josh Earnest said U.S. intelligence analysts have not determined when or how she died. 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," said Earnest.
He said, "What is not possible to call into question is that ISIL, regardless of her cause of death, is responsible for it."
Earlier Tuesday, National Security Council Spokesperson Bernadette Meehan said Mueller's family received a "private message" from her captors in recent days with additional information that corroborated news of her death.
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," Kerry said.
Mueller's aunts tearfully spoke of their niece to reporters later Tuesday.
"She had a quiet, calming presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice," Lori Lyon and Terri Crippes said, reading from a prepared statement. Her childhood friend Kathleen Day said, "We just delighted in that, that Kayla remained Kayla. Now she's free... She says that she found freedom even in captivity and that she is grateful, and so her light shines."
U.S. Senator John McCain, who represents Mueller's home state of Arizona, said Tuesday "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 said in a statement they were "heartbroken" over their daughter's death.
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.
VOA's Jeff Seldin contributed to this report.