The parents of an American woman held by the Islamic State group said in a statement on Tuesday that they have been notified of her death.
The White House said Kayla Jean Mueller's family received a message from her captors over the weekend, which was authenticated by the U.S. intelligence community, the Associated Press reports.
On Friday, the Islamic State group released a statement claiming Mueller had been killed in a Jordanian airstrike in Raqqa, Syria. The Jordanian government said later that it was "highly skeptical" about the extremists' statement.
Mueller, who is from Prescott, Arizona, was captured by militants in the northern Syrian city of Aleppo in August 2013. U.S. officials acknowledged last year that a 26-year-old American woman was being held by the group, but did not identify her out of fears for her safety.
Mueller's family honored her dedication to humanitarian work in their statement on Tuesday, according to The Arizona Republic. "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," the statement read.
"Our hearts are breaking for our only daughter, but we will continue on in peace, dignity, and love for her," the family said.
The family also released a letter that Mueller wrote while in captivity in 2014, BuzzFeed notes. "I know you would want me to remain strong. That is exactly what I'm doing," Mueller wrote. "[I] have learned that even in prison, one can be free. I am grateful. I have come to see there is good in every situation, sometimes we just have to look for it."
President Barack Obama sent his condolences to the family on behalf on the American people in a statement on Tuesday.
"In how she lived her life, she epitomized all that is good in our world," he said. "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."
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.
A 2007 article about Mueller from the same newspaper said she was a student at Northern Arizona University and was active in the Save Darfur Coalition. A statement from the office of Sen. John McCain, R-Ariz., said Mueller graduated in 2009 and had worked to help people in need in India, Israel, the Palestinian territories and in Arizona.
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.
Read the Mueller family statement below:
"We are heartbroken to share that we've received confirmation that Kayla Jean Mueller, has lost her life.
"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 to her father on his birthday in 2011, Kayla wrote:
'I find God in the suffering eyes reflected in mine. If this is how you are revealed to me, this is how I will forever seek you.'
'I will always seek God. Some people find God in church. Some people find God in nature. Some people find God in love; I find God in suffering. I've known for some time what my life's work is, using my hands as tools to relieve suffering.'
"Kayla was drawn to help those displaced by the Syrian civil war. She first traveled to Turkey in December, 2012 to provide humanitarian aid to Syrian refugees. She told us of the great joy she took in helping Syrian children and their families.
"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.
"Our hearts are breaking for our only daughter, but we will continue on in peace, dignity, and love for her.
"We remain heartbroken, also, for the families of the other captives who did not make it home safely and who remain in our thoughts and prayers. We pray for a peaceful resolution of the conflict in Syria."