Prosecutors say police have arrested family members of one of the Paris attackers – a French citizen named by the Mayor of Chartres as Ismael Omar Mostefai.
Paris prosecutor’s office spokeswoman Agnes Thibault-Lecuivre declined to give further details of how many people had been detained, but said searches were under way.
Mostefai, 29, was one of the eight suspects killed in the series of shootings and bombings at restaurants, bars, a music venue and the national football stadium.
He was born in Courcouronnes, Essonne, on 21 November 1985, and lived in Chartres, southwest of Paris.
Paris prosecutor Francois Molins said he had a criminal record and was known to security services but had not spent time in jail.
Mr Molins said: “He caught police’s attention due to the violation of public power. From 2004 to 2010, he was pronounced guilty eight times, but has never been in prison.
“In 2010, he was blacklisted by the police due to extreme behaviours, but never been classified into any illegal extremist groups.”
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Three teams of attackers in identical explosives vests seem to have co-ordinated the “act of barbarism” that left 129 people dead and 352 injured across the French capital, Mr Molins said.
Three other people, including a French citizen, have been arrested in connection with the atrocities. They were detained at the Belgian border.
Three others were arrested in police raids in Molenbeek, a poor, immigrant area of Brussels.
Belgian Prime Minister Charles Michel said at least one of those held in Molenbeek was thought to have spent the previous night in Paris.
Two cars registered in Belgium were impounded close to scenes of some of the violence in Paris, including the Bataclan theatre.
Islamic State has claimed responsibility for the near-simultaneous attacks and has warned that France would remain at the “top of the list of targets” over its airstrikes on IS territory in Syria and Iraq.
It said in an online statement that eight militants armed with explosive belts and guns chose targets “in the capital of adultery and vice”.
French President Francois Hollande earlier said the assaults were an “act of war” by IS.
Syrian and Egyptian passports were found near the bodies of two of the attackers, with Greek officials suggesting that two of the suspects may have arrived in the European Union through Greece in recent months.
Mr Molins confirmed the Syrian passport found at the site of the Stade de France bombing belonged to a Syrian citizen born in 1990, but he was unknown to the security services.
Tony Smith, a consultant on global border control, said it is extremely difficult for the EU to track people once they are in Europe because there are no internal borders.
He told Xul News: “The complexion of this group suggests that there have been some breaches of the external EU border, and there aren’t any internal borders at the moment in Europe, so this is a very worrying development.
“What should happen is a very thorough identification and screening process at the first point entry but the sheer numbers – with a four-fold increase in migrants entering the EU irregularly in the first 10 months of this year over last year, which was a record number – means it is extremely difficult for border agencies, in places like Greece and Italy, to do all of those checks.
“Once people are on shore then they are free to roam around the Schengen group without further challenge.”
French Prime Minister Manuel Valls has vowed to “destroy” those behind the killings.
“We will strike this enemy to destroy him. In France and in Europe, we’ll chase the authors of this act, and also in Syria and Iraq. We will win this war,” he said.
Thousands of soldiers have been deployed on the country’s streets in the aftermath of the attacks.