When a graphic video surfaced this weekend, claiming to show that ISIS militants had beheaded the American aid worker Peter Kassig, investigators across the world quickly took notice: several knife-wielding men could be seen in the video, standing over the corpses of their captives, and the militants' faces were clearly visible.
By Monday, French officials said they had already identified one of the men as Maxime Hauchard, a French citizen from Normandy, who joined the extremist group last year. In July, Hauchard, who is believed to be around 22, gave an interview to France's BFM television, saying he had helped the militant group capture the strategically important city of Mosul in northern Iraq.
The man believed to be Hauchard can be seen on the right in the screenshot below. He appears in the video moments after the radicals behead at least 17 men, which ISIS in the video identify as Syrian soldiers.
Many ISIS militants are believed to hail from the Middle East, though investigators believe that the radical group has successfully recruited members among disaffected Muslims in Europe.
European officials have tried to counter Islamic State propaganda that encourage young western Muslims to take up arms.
"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 the Islamic State group. "These are criminals that are building a system of barbarity."
A British national with a distinct London accent also appears in the video. The masked man, who has been dubbed 'Jihadi John' in the media, also appeared in other videos showing the beheading of American hostages James Foley and Steven Sotloff, and British hostages David Haines and Alan Henning.
British intelligence officials have identified Abdel-Majed Abdel Bary, a 24-year-old Londoner, as a prime suspect. New video emerged on Monday showing Bary ranting against the police during a 2011 interview about the London riots with Australian 9News foreign correspondent Peter Stefanovic. Bary appears agitated and gestures wildly while denouncing the authorities.
"The police are protecting the police station, they're not protecting the people," he says in the interview, seen below.
Another man seen in the video was initially identified by his father as 20-year-old British man named Nasser Muthana. His father later said the man was not his son.
Many of the fighters in the video, however, appear to be foreign.
British Prime Minister David Cameron called Kassig's killing a "cold-blooded murder."
Even before the video surfaced, Cameron outlined new anti-terror laws. Under the laws, expected to be adopted later this month, British citizens suspected of fighting for ISIS could have their passports cancelled and their names added to a no-fly list.
I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL have again shown their depravity. My thoughts are with his family.— David Cameron (@David_Cameron) November 16, 2014
The Associated Press contributed to this report