WASHINGTON — Kayla Jean Mueller, a 26-year-old American woman held by Islamic State militants, has been confirmed dead, her parents and the Obama administration said Tuesday.
Mueller's family received a private message from her captors over the weekend and the information contained in that communication was authenticated by the U.S. The White House said the U.S. intelligence community has not been able to determine how or when she died.
"We are heartbroken to share that 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."
President Barack Obama said that Mueller, an aid worker who assisted humanitarian organizations working with Syrian refugees, "epitomized all that is good in our world."
"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," the president said.
The White House said Obama had spoken with Mueller's parents and offered his condolences and prayers.
Mueller, of Prescott, Arizona, is the fourth American to die while being held by Islamic State militants. Three other Americans — journalists James Foley and Steven Sotloff, and aid worker Peter Kassig — were beheaded by the group.
Journalist Austin Tice, of Houston, Texas, disappeared in August 2012 while covering Syria's civil war. It's not clear what entity is holding him, but it is not believed to be the Islamic State group or the Syrian government, his family has said.
Mueller was taken into captivity in August 2013 while leaving a hospital in Syria. Her identity was long kept secret out of fears for her safety.
On Friday, the Islamic State had claimed that Mueller died in a Jordanian airstrike. Jordan, which has launched a barrage of strikes in recent days in retaliation for the gruesome killing of one of its pilots at the hands of the militants, disputed that report.
In the U.S., there was growing certainty that the claim from IS was false. White House spokesman Josh Earnest said that Jordan's airstrikes had targeted an IS weapons compound near the group's stronghold of Raqqa in northern Syria that had been targeted before, and that there was no evidence that civilians were in the area ahead of the strike.
Added a U.S. intelligence official: "She was not killed in that airstrike."
Mueller's parents released a letter Tuesday that their daughter had written them while in captivity. In the undated letter, Mueller said she was, "in a safe location, completely unharmed."
"I am also fighting from my side in the ways I am able + I have a lot of fight left inside of me," she wrote. "I am not breaking down + I will not give in no matter how long it takes."
Secretary of State John Kerry said the U.S. was "unshaken" in its resolve to defeat the Islamic State, a group he called an "ugly insult to the civilized world."
Republican Sen. John McCain, who represents Mueller's home state of Arizona, said, "The thoughts and prayers of the people of Arizona, America, and the civilized world are with the Mueller family at this terrible hour."
Mueller's family said she graduated from Northern Arizona University in 2011 after just two-and-a-half years. In December 2012, she traveled to the Turkish-Syrian border to work with several humanitarian groups, including Support to Life and the Danish Refugee Council.
Mueller had also worked for aid groups in India, Israel and the Palestinian territories, as well as at an HIV/AIDS clinic and women's shelter in Arizona.
"We are so proud of the person Kayla was and the work that she did while she was here with us. She lived with purpose, and we will work every day to honor her legacy," Mueller's family said.
AP Intelligence Writer Ken Dilanian contributed to this report.