Islamic State militants have beheaded Peter Kassig, a U.S. aid worker and former Army Ranger who was captured last year in Syria while on a private humanitarian mission, the White House confirmed Sunday, Nov 16, Los Angeles Times reported.

U.S. officials authenticated a 16-minute video, released on various social networks by the militant group earlier in the day. The video also includes footage of the mass beheading of men said to be Syrian government soldiers.

Kassig, 26, became the third U.S. citizen known to have been executed by Islamic State, an Al Qaeda offshoot that controls vast swaths of territory in Syria and neighboring Iraq. The group arose amid the tumult of the Syrian conflict, now in its fourth year.

President Obama condemned his killing as an “act of pure evil by a terrorist group that the world rightly associates with inhumanity.”

Islamic State has previously released videos of the beheading of two U.S. journalists, James Foley and Steven Sotloff. The militants have also beheaded two British aid workers, David Haines and Alan Henning.

Kassig adopted the Arabic name after converting to Islam while in custody, his family said. His parents refer to him as Abdul-Rahman on a Facebook page dedicated to his release.

“While ISIL revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction, Abdul-Rahman was a humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict,” Obama said.

Islamic State demonstrated that it represented no faith, he said, “least of all the Muslim faith which Abdul-Rahman adopted as his own.”