Terrorist group Islamic State (ISIS) has beheaded Peter Kassig, an American aid worker and former soldier, making him the fifth Westerner to be beheaded by the widely reviled Islamic militant group.

The White House confirmed on Sunday that Kassig, who took up the name Abdul-Rahman Kassig and converted to Islam before being taken hostage by ISIS in 2013, was beheaded. President Barack Obama himself confirmed Kassig's decapitation hours after a 16-minute video made the rounds of the Internet claiming to be made by the terrorist group.

In a statement issued from Air Force One, the President offered his prayers and condolences to the friends and family of Kassig, who was "taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity."

"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 says. "While ISIL exploits the tragedy in Syria to advance their own selfish aims, Abdul-Rahman was so moved by the anguish and suffering of Syrian civilians that he traveled to Lebanon to work in a hospital treating refugees."

Other groups, notably the Council on American-Islamic Relations (CAIR), a Washington-based Muslim-advocacy group recently branded by the United Arab Emirates as a terrorist organization, condemned Kassig's "barbaric murder."

"Just as we have denounced previous killings of innocent civilians by the ISIS terrorist group, we condemn the barbaric murder of Peter Kassig and once again repudiate the anti-Islamic ideology that produces such brutality," says CAIR in a statement. "We also offer our sincere condolences to Mr. Kassig's family and loved ones."

The video does not explicitly show footage of Kassig, unlike in earlier ISIS videos where Western hostages were first shown declaring statements denouncing their governments while clearly under duress. Matthew Olsen, former director of the National Counterterrorism Center, suggests that Kassig may have refused to denounce the U.S. government, explaining his lack of appearance in the video.

However, the video does show what appears to be Kassig's severed head sitting on the desert sands of Dabiq in Aleppo province, where the militants claim the video was shot. Footage of Kassig's head appears toward the end of the video, with Jihadi John, the same masked British jihadist that had appeared in previous decapitation videos declaring that the head was indeed Kassig's.

"This is Peter Edward Kassig, a U.S. citizen," the black-clad militant with the British accent says as he stood over the severed head. "Peter, who fought against the Muslims in Iraq while serving as a soldier under the American Army, doesn't have much to say."

Earlier in the video, several uniformed ISIS militants are shown beheading more than a dozen members of the Syrian air force in synchronized fashion, with Jihadi John taunting the U.S. and U.K. governments and threatening to "slaughter your people on your streets."

"To Obama, the dog of Rome. Today, we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers," he says. "And with Allah's permission, we will break this final and last crusade. And the Islamic State will soon, like your puppet David Cameron said, begin to slaughter your people on your streets."

American journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning were previously beheaded by ISIS, which claims that the murders were done in revenge of the U.S.-led air and ground strikes against ISIS militants in Syria and Iraq. 

Kassig was last seen alive towards the end of the video that showed the beheading of Henning. The 26-year-old aid worker was captured in October last year as he was traveling in an ambulance to Deir Ezzor in eastern Syria. Following the ISIS threat that Kassig was next in line for decapitation, Kassig's parent's Ed and Paula released their own YouTube video pleading ISIS to spare their son's life.

In 2006, Kassig served in the Army's 75th Ranger Regiment and was deployed to Iraq in 2007 before being honorably discharged as a private first class for medical reasons and enrolling in Butler University in Indianapolis to study political science and join the track team. He was, however, unhappy with his situation.

"I was going to school with kids who look the same, were the same age as me, but we weren't the same," Kassig told CNN in 2012. "I wanted more of a challenge, a sense of purpose."

During the spring break of 2012, Kassig flew to Lebanon to volunteer as an aid worker serving Pakistani refugees and other war victims. With his own savings and donations from family and friends, he later founded the Special Emergency Response and Assistance (SERA), an aid organization that delivered food, medical supplies and other aid-related supplies to war-stricken areas in Syria. As a fully trained emergency medical technician himself, Kassig led SERA volunteers to teach medics and civilians first aid and trauma training.

"We know he found his home amongst the Syrian people, and he hurt when they were hurting," say Kassig's parents. "We are incredibly proud of our son for living his life according to his humanitarian calling."

Kassig was fully aware of the dangers of pursuing his calling and knew that it was highly likely that he might not be freed from captivity alive. In a letter written to his parents, parts of which they released to the public, Kassig offered words of comfort to his grieving parents.

"I am very sad that all this has happened and for what all of you back home are going through," he wrote. "If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need."

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