Last Updated Feb 10, 2015 4:25 PM EST
WASHINGTON -- Kayla Jean Mueller, a 26-year-old American woman held by Islamic State of Iraq and Syria (ISIS) militants, has been confirmed dead, her parents and the Obama administration said Tuesday.
CBS News national security correspondent David Martin reports that ISIS sent the family an e-mail telling them their daughter was dead and including a picture. The picture showed that she had died "recently" but the cause and exact date of her death were inconclusive. The U.S. intelligence community was then able to verify the photo.
"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, "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 ISIS 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 ISIS 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, ISIS 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 ISIS was false. White House spokesman Josh Earnest said that Jordan's airstrikes had targeted an ISIS 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.
Navy Rear Adm. John Kirby was asked Tuesday if there was any doubt who killed the aid worker. He replied: "No doubt. ISIL," using another acronym the group is known by.
Mueller had been working in Turkey assisting Syrian refugees, according to a 2013 article in The Daily Courier, her hometown newspaper. She told the paper that she was drawn to help with the situation in Syria.
"For as long as I live, I will not let this suffering be normal," she said. "It's important to stop and realize what we have, why we have it and how privileged we are. And from that place, start caring and get a lot done."
According to the newspaper, Mueller had been working with the humanitarian aid agency Support to Life, as well as a local organization that helped female Syrian refugees develop skills.
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 father briefly had a glimmer of hope that his daughter might be rescued but was mostly overwhelmed by "devastation and hopelessness" during the year and a half that his daughter was held captive by ISIS, a family friend told CBSN on Tuesday.
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's family said, "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 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."