Friends and colleagues of Kayla Mueller, the aid worker who died while a captive of militants of the Islamic State group in Syria, remembered her on Saturday as someone who was trying to give back in gratitude for a life of freedom.

About 150 people gathered for a candlelight vigil in Mueller's honor Saturday evening at the United Christian Ministry at Northern Arizona University in a tearful and joyous celebration of her life.

"She was in a volatile part of the world because she knew there were things that needed to be addressed there," said Peggy Sheldon-Scurlock, who met Mueller at the New Day Peace Center in Flagstaff, a non-profit peace organization. "She was all about action."

Mueller, 26, was confirmed to have died under circumstances that remain unclear about 18 months after she was abducted while leaving a hospital in northern Syria.

The White House said Mueller's family received a message from her captors containing information that was authenticated by U.S. intelligence analysts who verified her death.

President Barack Obama also acknowledged in an interview with the website Buzzfeed that Mueller was among the hostages whom U.S. commandos were sent to rescue but failed to find in an operation he ordered last year.

In Flagstaff on Saturday, the Rev. Kathleen Day, head of the United Christian Ministry at Northern Arizona University, said Mueller wanted to help people and she felt a responsibility to maximize the privileges she had as a resident of the United States.

"When she saw what was happening in Syria, that situation was so overwhelming, it was just crushing," Day said. "For her, it was a calling."

Her friend Emily Waldron said Mueller was humble and unselfish.

"Even though there's this cause that she lost her life in that's so big, I think it is important to know that she did little acts of kindness every single day," Waldron said.

Last Wednesday, a day after Mueller was reported dead, Obama sent Congress a request to authorize military force against Islamic State, saying the group "has committed despicable acts of violence and mass execution."

Its militants have killed thousands of civilians while seizing territory in Iraq and Syria in an attempt to establish a hub of jihadism in the heart of the Arab world.

They have also generated international outrage by beheading western aid workers and journalists and burning to death a Jordanian pilot.