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.
The White House said that Mueller's family received a private message from her captors over the weekend and the information contained in that communication was authenticated by the US intelligence community. It was not immediately clear how and when Mueller 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, who was an aid worker who assisted humanitarian organizations working with Syrian refugees, "epitomised 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 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 and the White House said at the time that it had seen no evidence to corroborate the Islamic State claims.
Secretary of State John Kerry said the US was "unshaken" in its resolve to defeat the Islamic State, a group he called an "ugly insult to the civilised world."
Republican senator 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. 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.
At least one other American was being held hostage in the Middle East, White House spokesman Josh Earnest told reporters on Tuesday following confirmation of the death of Mueller.
"We have avoided discussing the individual cases of Americans who have been held hostage, but we are aware of other American hostages being held in the region," Earnest said a briefing, in response to a question about whether Islamic State militants were holding any other Americans hostage.