Peter Kassig, the U.S. aid worker beheaded by Islamic State militants, was remembered Sunday for his devotion to helping Syrians. President Obama praised Kassig as a humanitarian killed in “an act of pure evil.” (Reuters)

The Islamic State beheaded an American Muslim convert and former U.S. Army Ranger who had traveled to Syria on a humanitarian mission, according to a gruesome video released Sunday by the terrorist group.

The death of Abdul-Rahman Kassig, previously known as Peter, came weeks after the group said it would kill him because of the U.S. bombing campaign in Syria.

“Today we offer our prayers and condolences to the parents and family” of Kassig, President Obama said in a statement released Sunday afternoon. “We cannot begin to imagine their anguish at this painful time.”

“Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity,” Obama said.

The highly choreographed video, which the White House confirmed was authentic, shows a masked militant with a British accent saying: “This is Peter Edward Kassig, a U.S. citizen of your country. Peter, who fought against the Muslims in Iraq while serving as a soldier under the American army, doesn’t have much to say.” 

Kassig, 26, is the fifth Western hostage and third American whose killing by the Islamic State has been depicted or announced in a video. The new video does not show the militant beheading Kassig. Near the end, Kassig’s head appears in the bottom of the frame. His body can’t be seen. In previous videos, hostages wearing orange jumpsuits had made statements while a British jihadist stood behind them with a knife in his hand. The militant would then kill the hostage, though the footage did not show the actual decapitation.

However, the latest video — the longest so far, running more than 15 minutes — does show at least a dozen men, apparently Syrian soldiers, being decapitated simultaneously. The terrorist group said the video was made in Dabiq, in Syria’s Aleppo province.

The militant in the latest video appears to be the same one who carried out the previous beheadings. U.S. and British authorities have identified the man but have not released any information about him to the public.

Kassig was detained Oct. 1, 2013, in eastern Syria while traveling in an ambulance.

In a statement Sunday, his family said, “We are heartbroken to learn that our son, Abdul-Rahman Peter Kassig, has lost his life as a result of his love for the Syrian people and his desire to ease their suffering. Our heart also goes out to the families of the Syrians who lost their lives, along with our son.”

Kassig’s family had appealed to the Islamic State not to kill their son. The family also has tried to sway the group with its repeated statements about his conversion to Islam.

His family said Kassig, who was raised in Indiana, converted to Islam last year while sharing a cell with a devout Syrian Muslim. The family said the conversion process started before he was taken hostage.

“ISIL’s actions represent no faith, least of all the Muslim faith which Abdul-Rahman adopted as his own,” Obama said in his statement about the killing, using another name for the Islamic State.

Secretary of State John F. Kerry said in a statement that Kassig “was a young American who personified the values of altruism and compassion which are the very essence of his adopted religion of Islam.”

Before he was killed, Kassig’s family released a letter he had written earlier this year that a freed hostage had brought with him. The family also received an audio recording of their son before the Islamic State revealed that it was holding him hostage.

“I am obviously pretty scared to die but the hardest part is not knowing, wondering, hoping, and wondering if I should even hope at all,” Kassig wrote. “I am very sad that all this has happened and for what all of you back home are going through. 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.”

The Islamic State is also holding a 26-year-old American woman and another woman. The American woman was kidnapped while doing humanitarian work in Syria.

Her family and the FBI have asked that her name not be released for fear of putting her in greater danger.

In addition to two British aid workers, the Islamic State has killed American journalists James Foley and Steven Sotloff.

Kassig traveled to Lebanon while on spring break from Butler University in March 2012 to work as a volunteer emergency medical technician. Then, several months later, he founded Special Emergency Response and Assistance (SERA), a nongovernmental organization.

In summer 2013, he moved its base of operations to Gaziantep, Turkey.

Kassig enlisted as an infantryman and was in the Army from June 2006 to September 2007 before being medically discharged as a private first class, Army officials said.

He was with the Army’s famous 75th Ranger Regiment from October 2006 to September 2007 and deployed with the regiment to Iraq from April to July 2007.

Although other countries have paid ransoms to free their citizens, the U.S. and British governments have steadfastly refused to do so.

“They tell us you have abandoned us and/or don’t care but of course we know you are doing everything you can and more,” Kassig wrote in the letter his family released.

In a recent video, one of the hostages still being held, British journalist John Cantlie, said the terrorist group had started capturing Westerners entering Syria in 2013.

Cantlie said the Islamic State had freed 16 people from six European countries after their governments negotiated for their release.

Cantlie also said the Islamic State had killed a Russian man.

Katie Zezima and Carol Morello contributed to this report.