Peter Kassig went to Syria to provide relief for refugees fleeing the ongoing violence, but now finds himself the next ISIS hostage threatened with death.

He was shown in a video released Friday by Islamic State of Iraq and Syria (ISIS) militants in which hostage Alan Henning was apparently beheaded. At the end of the video, the masked, English-speaking executioner warned that Kassig, kneeling next to him, would be next.

The 26-year-old Indiana native, who now goes by the name Abdul-Rahman, according to his family, is a former U.S. Army Ranger who served in Iraq in 2007.

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After returning home from the service, he began to study political science at Butler University, but took time off in 2010 to get certification as an emergency medical technician, according to a CNN profile. In the next two years, he got married but soon divorced.

He went back to school, and during spring break in 2012, he volunteered as a medical worker in Lebanon helping Palestinian and later Syrian refugees.

Later that year, Kassig started a non-governmental organization called Special Emergency Response and Assistance (SERA) to respond to the needs of Syrians attempting to escape the violent conflict.

Peter Kassig, the American founder of the relief organization Special Emergency Response and Assistance (SERA) and Iraq War veteran in an undated photo. SERA

But the focus of his operations, to provide aid in areas of Syria, Lebanon and Turkey that would be too hard for other humanitarian organizations to enter, left him vulnerable. He went missing in October 2013 on his way to Deir Ezzour in eastern Syria and at some point ended up in the hands of ISIS militants.

In a 2012 post on his Fundrazr.com website, he explained what motivated him to work in the war-torn region.

"I felt that more could be done to help people and that the most effective way to bring assistance was through a close connection to those who were in desperate need, by meeting them where they were," Kassig wrote on the website. "I also felt that through a personal relationship and a small yet efficient organization, I could offer people a high level of transparency and integrity in the ways I used funding and the activities that SERA engages in to help people."

In a statement released Friday, Kassig's family requested privacy and extended their condolences to Henning's loved ones:

"The Kassig family extends our concern for the family of Alan Henning. We have read about his work and his generous character with great respect and admiration. We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe."