The Islamic State beheaded a Muslim convert and former U.S. Army Ranger who had traveled to Syria on a humanitarian mission, according to a purported video of the execution released Sunday.
The death of Abdul-Rahman Kassig, previously known as Peter, came weeks after the terrorist group said it would kill him because of the U.S. bombing campaign in Syria.
The video shows a militant with a British accent standing over the young man's severed head and 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."
The video doesn't depict the militant beheading Kassig.
The militant 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.
A spokeswoman for the National Security Council said intelligence officials were trying to determine the authenticity of the video.
"If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," NSC's Bernadette Meehan said.
Kassig, 26, is the fifth Western hostage whose apparent killing by the Islamic State has been announced in gruesome videos. The latest video was the longest in length, running more than 15 minutes and showed other men, apparently Syrian soldiers, being decapitated.
Kassig was detained Oct. 1, 2013, in eastern Syria while traveling in an ambulance.
In a statement Sunday, his family said, "We are aware of the news reports being circulated about our treasured son and are waiting for confirmation from the government as to the authenticity of these reports. We will have no other statement at this time and ask that you please respect our privacy."
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.
Before he was killed, Kassig's family released a letter he had written earlier this year that a released hostage had carried out. 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.