According to international news agencies, Mueller's family received an email and photograph over the weekend from her captors that enabled American intelligence to determine that she had been killed, U.S. officials said.
The circumstances of Kayla’s death are unclear, and U.S. President Barack Obama vowed to hunt down the culprits.
Islamic State claims that Mueller was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold in Syria of the Islamist militant group.
Jordan and U.S. officials have expressed doubt about Islamic State's account of her death.
According to a family spokesperson Kayla Mueller has been held by the terrorist group for nearly two years, and was abducted August 4, 2013, while serving on a humanitarian mission in the region. She was returning from a Doctors Without Borders facility at the time of her abduction.
She had been working primarily with women and children in the region, delivering humanitarian aid to those displaced by the Syrian conflict.
ISIS had previously demanded $6.6 million for her release, but the American and British governments have refused to pay ransoms for their citizens.
After graduating in 2010, she volunteered at Food For Life Vrindavan (FFLV) for 6 months. According to the tribute FFLV has put up on their website, “at the beginning she was giving English classes and then the kindergarten kids stole her heart. She was very simple and humble. She loved the kids, without the “fear” that many foreigners have. She made many friendships with the parents of the children, used to visit them in the slum, even eat with them. After her time here, she went back to USA to make some money and went to Darfur, Sudan, into the civil war. After, again went back to USA to make more money and left for the Middle East… She loved those people… Our prayers go out to her family and friends, a wonderful soul who was making a real difference in many people’s lives. Sure she has gone on to a better life in the spiritual realm.”
Kayla Mueller's family quoted from a letter she sent her father on his birthday in 2011: "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."
