Hours later, the White House confirmed Kassig’s death after a review of the video, which also showed the mass beheadings of a dozen Syrian soldiers.

The 26-year-old Kassig, who returned to the Middle East to help wounded and displaced Syrians, “was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity,” President Barack Obama said in a statement.

He denounced the extremist group, which he said “revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction.”

With Kassig’s death, the Islamic State group has killed five Westerners it was holding. American journalists James Foley and Steven Sotloff were beheaded, as were British aid workers David Haines and Alan Henning.

Unlike previous videos of slain Western hostages, the footage released Sunday did not show the decapitation of Kassig or the moments leading up to his death.

“This is Peter Edward Kassig, a U.S. citizen … who fought against the Muslims in Iraq,” said the black-clad militant, who spoke with a British accent that was distorted in the video, apparently to disguise his identity. Previous videos featured a militant with a British accent that the FBI says it has identified, though it hasn’t named him publicly.

The slain hostage’s parents, Ed and Paula Kassig, said they were “heartbroken” by their son’s killing, but “incredibly proud” of his humanitarian work. Kassig “lost his life as a result of his love for the Syrian people and his desire to ease their suffering,” the parents said in a statement from Indianapolis.

The high-definition video released Sunday also showed the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The black-clad militant warns that U.S. soldiers will meet a similar fate.

“We say to you, Obama: You claim to have withdrawn from Iraq four years ago,” the militant said. “Here you are: You have not withdrawn. Rather, you hid some of your forces behind your proxies.” A U.S.-led coalition is targeting the Islamic State group in airstrikes, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

Kassig, who served in the U.S. Army’s 75th Ranger Regiment, a special operations unit, deployed to Iraq in 2007. After being medically discharged, he returned to the Middle East in 2012 and formed an aid organization, Special Emergency Response and Assistance, to aid Syrian refugees.

A certified EMT, Kassig had delivered food and medical supplies and provided trauma care to wounded Syrians before being captured in eastern Syria on Oct. 1, 2013. Friends say he converted to Islam in captivity and took the first name Abdul-Rahman.

In a statement issued as he flew back to Washington after a trip to the Asia-Pacific region, Obama said Kassig “was a humanitarian who worked to save the lives of Syrians injured and dispossessed” by war. The president offered prayers and condolences to Kassig’s family. “We cannot begin to imagine their anguish at this painful time,” he said.

Earlier, Kassig’s family said that it preferred “our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause.”

British Prime Minister David Cameron said he was “horrified by the cold-blooded murder,” saying that the Islamic State group had “again shown their depravity.”

The Islamic State group still holds other captives, including British photojournalist John Cantlie, who has appeared in several videos delivering statements for the group, likely under duress, and a 26-year-old American woman captured last year in Syria while working for aid groups. U.S. officials have asked that the woman not be identified out of fears for her safety.