French police have identified three teams of gunmen wearing suicide vests who killed at least 129 people during an onslaught in Paris.
A 29-year-old Frenchman was the first to be confirmed among the seven attackers, all of whom died in the assault, while two other men who registered as refugees in Greece have been linked to the atrocities.
The Greek line of inquiry, and the discovery of at least one Syrian passport at the scene, has raised suspicions some of the attackers or accomplices might have entered Europe as part of an influx of people fleeing Syria's civil war.
"A Syrian passport in the name of a person born in Syria in September 1990 was found near a suicide bomber who blew himself up at the Stade de France (stadium)," prosecutor Francois Molins told a press conference on Saturday (local time).
Earlier in the day, the Islamic State extremist group claimed responsibility for the attacks that left a trail of destruction at a sold-out concert hall, at restaurants and bars, and outside France's national stadium.
A total of 129 people were killed and 352 wounded, of whom 99 were said to be in critical condition. The toll does not include the seven attackers.
President Francois Hollande called the co-ordinated assault on Friday night an "act of war" as the Paris capital's normally bustling streets fell eerily quiet.
A national emergency and three days of mourning have been declared. Paris City Hall ordered all city facilities closed, while the Eiffel Tower shut indefinitely.
Amid the fear there was also defiance, as hundreds gathered spontaneously at blood donation centres and flocked to place candles and flowers at the sites where people had died only hours earlier.
The remains of the 29-year-old French gunman were found near the Bataclan concert hall in eastern Paris, the bloodiest of the attacks.
A total of 89 people were killed there by armed men who burst in shouting "Allahu akbar" (God is great) before gunning down concert-goers and executing hostages.
The jihadists were heard raging at the French president and his decision in September to begin air strikes on Islamic State in Syria.
The investigation quickly spread beyond France on Saturday as Belgian police arrested several suspects in Brussels, including one who was in Paris at the time of the carnage.
Michel told Belgian television that the arrests were linked to "suspect vehicles" identified during the investigation by French police.
French authorities also asked their Greek counterparts for information on the passport found at the scene of the Stade de France attack.
"We confirm that the (Syrian) passport holder came through the Greek island of Leros on October 3 where he was registered under EU rules," said a statement issued by Greek minister for citizen protection, Nikos Toskas.
Another foreigner sought in connection with the attacks was also found to have registered in Greece, police sources confirmed.
Greece is the main entry point in Europe for hundreds of thousands of Syrians refugees and European security forces have long feared that IS militants could hide among them.
In Germany, the authorities said they were looking into a possible link between the attacks and the arrest in Bavaria last week of a man with a car-load of weapons and explosives.
The Paris attacks were "prepared, organised and planned overseas, with help from inside (France)", Hollande said.