The family of an American aid worker and former Army Ranger who was beheaded in a video released by Islamic State militants said on Monday their "hearts are battered, but they will mend."

Peter Kassig, 26, who also was known as Abdul-Rahman after he converted to Islam during his captivity, had been held by the Islamic State, also known as ISIS or ISIL, since October 2013 when he was abducted while delivering food and medical supplies to wounded Syrians in eastern Syria. A video showing his beheading was released on Sunday and later confirmed by U.S. officials.

"A while ago, we were informed that our beloved son, Abdul-Rahman, no longer walks this earth," Ed Kassig, Peter's father, said in a statement to reporters at Epworth United Methodist Church in Indianapolis, Ind. "Our hearts, though heavy, are held up by the love and support that has poured into our lives these last few weeks."

His mother, Paula Kassig, called Peter a "realist and idealist."

"Peter has earned the right to be both," she said, her voice at times quivering with emotion. "In 26 years, he has witnessed and experienced first hand more of the harsh realities of life than most of us can imagine. But, rather than letting the darkness overwhelm him, he has chosen to believe in the good –- in himself and in others. Peter’s life is evidence that he’s been right all along; one person can make a difference."

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The parents concluded the brief statement by asking for privacy to mourn and forgive, and asked for prayers for Kassig and others held "against their will in Syria, Iraq and around the world."

In the nearly 16-minute video uploaded to social networks, a black-clad militant with his face concealed stands before a severed head that he says is that of the U.S. aid worker.

President Obama confirmed the video's authenticity on Sunday, saying in a statement the act was "pure evil."

Kassig is the fifth Western hostage killed by ISIS in less than three months, and the third American. Previous Western beheading victims were American journalists James Foley and Steven Sotloff, as well as Britons David Haines, a former Royal Air Force engineer, and Alan Henning, a taxi driver from northwest England. The group is also holding British photojournalist John Cantlie, who has appeared in several other videos released by the group functioning as a de facto spokesman.

It is not clear when the Kassig video was filmed. Last month, a Twitter account linked to ISIS posted a message warning that Kassig had only days to live. Sources in the intelligence community told Fox News that the message was being tracked.

Kassig formed the aid organization Special Emergency Response and Assistance, or SERA, in Turkey to provide aid and assistance to Syrian refugees. He began delivering food and medical supplies to Syrian refugee camps in 2012 and is also a trained medical assistant who provided trauma care to injured Syrian civilians and helped train 150 civilians in providing medical aid.

Fox News' Catherine Herridge and The Associated Press contributed to this report.