France said one of its citizens is among the Islamic State militants who participated in killing American aid worker Abdul-Rahman Kassig.
Interior Minister Bernard Cazeneuve identified the Frenchman Monday as 22-year-old Maxime Hauchar, who left for Syria more than a year ago after a brief stay in Mauritania.
Authorities are also trying to determine whether a second French national was among the 16 jihadists who took part in the beheading of Kassig and at least 18 Syrian soldiers.
The British-based Syrian Observatory for Human Rights said the Islamic State group has now executed more than 1,400 people in Syria since the end of June, when it declared a caliphate over vast territories it controls in Iraq and Syria.
Taken captive in Syria
Kassig, 26, was taken captive 13 months ago while doing humanitarian work in war-torn Syria.
In a letter smuggled out by a fellow prisoner, Kassig had written to his parents about his possible fate.
He wrote: "If I do die, I figure 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."
President Barack Obama on Sunday condemned his killing, calling it "an act of pure evil."
Defense Secretary Chuck Hagel, who lauded Kassig for his service as an Army Ranger in Iraq, told reporters that Kassig had "extraordinary values and principles," and that applying them by helping others wherever it was needed "cost him his life."
Hagel said the United States is committed to defeat the Islamic State group.
"No one wants to live in a world of this kind of inhumanity and brutality, and all of us as human beings have a responsibility to see that that doesn't happen. It's difficult, but we will prevail,” Hagel said.
Hagel said in an earlier written statement that the killing shows the stark contrast between the "inhumanity" of the Islamic State fighters and the "bright and generous spirit" of Kassig.
Kassig, who witnessed the suffering of the Syrian people while visiting the region as a college student in 2012, was captured in eastern Syria in October 2013 while delivering relief supplies for Special Emergency Relief and Assistance, the aid group he founded.
He is the fifth Westerner to be beheaded by the Islamic State group. The militant Sunni group executed American journalists James Foley and Stephen Sotloff, and British aid workers David Haines, a taxi driver, and Alan Henning, a former Air Force engineer.
Kassig’s parents, Ed and Paula Kassig of Indianapolis, Indiana, who pleaded in a video last month to Islamic State militants to spare their son’s life, said they were heartbroken by the news.
In a statement Sunday, the Kassig family urged the media not to broadcast images released by the militants, saying this was "playing into the hostage-takers hands."
As with earlier beheading videos, the one released Sunday features a militant speaking in a British accent.
He said the militants are burying the "first American crusader" in the northern Syrian town of Dabig, and warns that U.S. soldiers will meet a similar fate.
British Prime Minister David Cameron said he was horrified by Kassig's "cold-blooded murder."