French citizen Omar Ismail Mostefai has been identified as one of the attackers who killed 129 people in Paris on November 13.
Omar Ismail Mostefai, 29, was named by local media and a French parliamentarian.
He had a criminal record and was known to have been radicalized.
Investigators identified Omar Ismail Mostefai after his severed fingertip was found at the Bataclan concert hall, where three attackers blew themselves up, AFP news agency reports.
Friday’s attacks, claimed by ISIS militants, hit a concert hall, a major stadium, restaurants and bars in the French capital.
Prosecutors say seven assailants – armed with Kalashnikovs and suicide belts – were organized into three teams, and there are fears that some may have fled the scene.
PM Manuel Valls has said France will continue with air strikes against ISIS in Syria, and described the group as a very well-organized enemy.
Police are trying to find out whether Omar Ismail Mostefai traveled to Syria in 2014, judicial sources told AFP.
His father and brother have been taken into police custody.
“It’s crazy, insane. I was in Paris myself last night, I saw what a mess it was,” Omar Ismail Mostefai’s older brother told AFP before being detained after voluntarily attending a police station on November 14.
Omar Ismail Mostefai came from the town of Courcouronnes, 15 miles south of Paris. He lived in the nearby town of Chartres until 2012, according to local lawmaker and deputy mayor Jean-Pierre Gorges.
He regularly attended the mosque in Luce, close to Chartres, AFP reported.
Omar Ismail Mostefai had a history of petty crime but was never jailed. The security services deemed him to have been radicalized in 2010 but he was never implicated in a counter-terrorism investigation.
His brother said he had not had contact with him for several years following family disputes, but said he was surprised to hear he had been radicalized.
He was one of six children in the family and had traveled to Algeria with his family and young daughter, the brother said.
The investigation is also focusing on a possible link to Belgium after police there arrested three men near the French border.
A black Volkswagen Polo with Belgian registration found at the Bataclan had been rented by a Frenchman living in Belgium, the Paris chief prosecutor said.
The French national was identified while driving another vehicle in a spot check by police on Saturday morning as he crossed into Belgium with two passengers.
Speaking in Paris, chief prosecutor Francois Molins told reporters: “We can say at this stage of the investigation there were probably three co-ordinated teams of terrorists behind this barbaric act.
“We have to find out where they came from… and how they were financed.”
Francois Molins said the police were also investigating a black Seat used by gunmen at two of the attacks, which remains untraced.
A Syrian passport, found near the body of one of the attackers at the Stade de France, had been used to travel through the Greek island of Leros last month, Greek officials have confirmed.
French President Francois Hollande has imposed a state of emergency after the worst peacetime attack in France since World War Two. It is also the deadliest in Europe since the 2004 Madrid bombings.