PARIS -- Three young Europeans are believed to be among the cold-eyed fighters on a propaganda video showing a beheaded American aid worker and the deaths of Syrian soldiers, as the Islamic State of Iraq and Syria (ISIS) tries to portray itself as an international movement.
President Barack Obama confirmed the slaying of American aid worker Peter Kassig after a U.S. review of the video, which also showed the mass beheadings of more than a dozen Syrian soldiers.
The overwhelming majority of ISIS fighters are from the Mideast, but the extremist group is trying to cement its claim on an Islamic empire straddling Iraq and Syria. Europe appears to be a fertile ground to find ISIS supporters, with officials saying thousands of young Europeans have headed off to jihad.
Francois Molins, a Paris prosecutor, said Monday that a French citizen who joined ISIS extremists in 2013 is be among the knife-wielding militants lined up behind their kneeling victims in the latest video.
The video camera lingered on the faces of Frenchman Maxime Hauchard and other extremists who seemed to have come from across the world. Some had distinctly Asian features, while another whose face was hooded had the familiar London accent of the jihadi who also appeared in beheading videos with American hostages James Foley and Steven Sotloff, and with British hostages David Haines and Alan Henning. There were also indications that a Welsh medical student may be the man standing next to Hauchard in the latest video.
Molins also said Monday that another Frenchman could be among the fighters in the video, but it is too early to determine for certain.
"It's quite transparent that (ISIS) is trying to exaggerate its base of support," said Charlie Winter, a researcher at the Quilliam Foundation in London. "They are trying to show that Muslims from all over the world are protecting their Syrian brethren and their Iraqi brethren."
European officials are trying just as furiously to counter that message.
"I call solemnly and seriously on all our citizens, and notably our young people who are the primary target of the terrorist propaganda, to open your eyes to the terrible reality of the actions of Daesh," Cazeneuve, the French minister said, using an Arabic acronym for ISIS. "These are criminals that are building a system of barbarity."
French Interior Minister Bernard Cazeneuve said authorities have been investigating Hauchard, who is around 22 and from west of Paris. The convert to Islam gave an interview to France's BFM television in July, telling the network he had helped capture Mosul, the Iraqi city whose fall eventually prompted the United States to resume military operations in Iraq.
"From a personal point of view, my objective is to be a martyr," Hauchard said in that interview.
One father from Wales, Ahmed Muthana, said he thinks he saw his son, 20-year-old Nasser Muthana, in the latest video, and a British researcher confirmed the likeness of the jihadi with the Cardiff medical student.
"It resembles him. I was shown a picture of the video. I cannot confirm it is him, but I think it might be," Ahmed Muthana told Britain's Press Association.
Obama denounced the extremist group, which he said "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction."
One 26-year-old former soldier from Holland told CBS News correspondent Carissa Ward last month why he joined the jihad in Syria two years ago.
"Look at this, please. It's getting bigger and bigger every day. No education, no proper food... nothing for these people," he said. "They live with the bare minimum. They've been begging for help for so long. And what do they get, they get bombs. And then they ask me, would you fight these people that are bombing them. I would fight anybody. Even if it was my own father that was bombing them, I would fight him and kill him myself."
The 26-year-old Kassig, who founded an aid group to help Syrians caught up in their country's brutal civil war, "was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said in a statement.
"(Our son) lost his life as a result of his love for the Syrian people and his desire to ease their suffering," the slain hostage's parents, Ed and Paula Kassig, said in a statement.
With Kassig's death, ISIS has killed five Westerners it was holding. 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, or threaten to kill any other Western hostages.
CBS News correspondent Margaret Brennan says ISIS is still believed to be holding at least one female American aid worker hostage, but right now her identity cannot be revealed. The group also holds British photojournalist John Cantlie, who has been shown in several videos delivering long statements in English on the group's behalf, perhaps under duress.
The footage released Sunday identified the militants' location as Dabiq, a town in northern Syria that ISIS uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will occur.
The high-definition video 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 ISIS in airstrikes in northern Syria, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military. The U.S. announced that a total of 31 airstrikes had been carried out from Nov. 14-17 against ISIS targets.
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 a relief group, Special Emergency Response and Assistance, to aid Syrian refugees.
The group's militants have beheaded and shot dead hundreds of captives, mostly Syrian and Iraqi soldiers, during its sweep across the two countries, and has celebrated its mass killings in extremely graphic videos.
ISIS has declared a self-styled Islamic caliphate in areas under its control, which it governs according to its violent interpretation of Shariah law, including massacring rebellious tribes and selling women and children of religious minorities into slavery.