French police have identified one of the gunmen who blew himself up at the Bataclan concert hall, the scene of the bloodiest attack where 89 people were killed, as 29-year-old Paris native Omar Ismail Mostefai.
His father and 34-year-old brother have been taken into custody by police and a source close to the probe said investigators are now searching the homes of other friends and relatives of the killer.
Mostefai, whose identity was confirmed using a severed fingertip, was known as being close to radical Islam, but had never been linked to terrorism.
Police said the attackers appeared to be "seasoned, at first sight, and well trained" and were investigating whether they had ever been to fight in Syria, where IS has proclaimed a caliphate along with territory in neighbouring Iraq.
The Paris attacks were "prepared, organised and planned overseas, with help from inside (France)," French President Francois Hollande said.
French police identified the first attacker out of the three teams of gunmen who carried out the worst ever attacks ever visited on Paris, which killed 129 people and wounded hundreds more.
The Islamic State group has claimed the carnage carried at some of the French capital's most popular night-spots, including a sold-out concert hall, at restaurants and bars and outside France's national stadium.
The seven attackers -- six blew themselves up and one was shot by police -- are the first to ever carry out suicide bombings on French soil and, unlike those who killed 17 in Paris in January, were unknown to security services.
Investigators in France, Belgium, Greece and Germany are now trying to find out who these men were, how they carried out such a vast coordinated attack, and why.
Belgian police have arrested several people over links to the Paris attacks in a huge sweep, including one who was in the French capital at the time of the attacks.
Justice Minister Koen Geens said the arrests were in connection with a grey Polo that had been rented in Belgium that was found near the Bataclan concert hall.
The arrests -- local media said three people had been detained -- took place in the poor Brussels district of Molenbeek that has been linked to several other terror plots in Europe.
Police in Belgium, which has the highest number of citizens per-capita who have gone to fight for IS in Europe, have opened a formal terrorism investigation.
Paris prosecutor Francois Molins separately said one of the vehicles used in Friday's attacks was registered in Belgium and hired by a French national living there.
Witnesses in Paris said some attackers arrived in a car with Belgian plates.