Isis militants have claimed that Kayla Jean Mueller, a 26-year-old US aid worker, has been killed in a retaliatory air strike conducted by Jordanian planes.
A statement from Isis (also known as Islamic State), apparently identifying the hostage, said she was killed during Muslim prayers, but it has not yet been independently verified.
Mueller, who is a native of Prescott, Arizona, reportedly demonstrated an interest in aid and human rights work during her high school years, as a member of the Save Darfur Coalition. She graduated from Northern Arizona University in Flagstaff in 2009 and afterwards worked at an orphanage in India, and at an HIV/AIDS clinic at home in the US.
She had volunteered for schools and aid organisations in both Israel and the West Bank, and in Dharamsala, India, where she taught English to Tibetan refugees.
According to the Washington Post, in 2012 Mueller began working with victims of the conflict in Syria for the humanitarian group Support to Life.
She disappeared after driving into the norther Syria city of Aleppo with her Syrian boyfriend and was captured along with several other aid workers in August 2013 while working at a hospital in Aleppo.
Her colleagues received subsequently received a proof-of-life video, showing her wearing a hijab and begging for her life.
A year later, CBS News reported that Isis had demanded a ransom equivalent to £4.3m for her release.
Her identity was never disclosed by authorities out of concern for her safety when she was taken hostage.
In 2013, Mueller spoke about her experiences to the Prescott Kiwanis Club, a volunteer group of which her father Carl is a member. “Syrians are dying by the thousands, and they're fighting just to talk about the rights we have,” she said.
“For as long as I live, I will not let this suffering be normal, something we just accept... It’s important to stop and realise what we have, why we have it and how privileged we are. And from that place, start caring and get a lot done.”
"When Syrians hear I'm an American, they ask, 'Where is the world?' All I can do is cry with them, because I don't know," she said.
A statement from her family's representative said: "The suffering of the Syrian refugees drew Kayla to the Turkish/Syrian border... to assist families who had been forced to flee their homes.
"Kayla found this work heartbreaking but compelling; she is extremely devoted to the people of Syria.
"The common thread of Kayla's life has been her quiet leadership and strong desire to serve others.
"When asked what kept her going in her mission, she said: 'I find God in the sufferng eyes reflected in mine. If this is how you are revealed to me, this is how I will forever seek you.'"