Police have begun questioning the relatives of the first terrorist to be identified after the attacks in Paris that left 129 people dead, French media reported as the investigation into the attacks widened to take in three other European countries.
The father and brother of Omar Ismail Mostefai, one of the seven jihadis killed in the deadliest attacks on French soil since the second world war, were among those being held, a judicial source confirmed.
Islamic State has claimed responsibility for the assaults.
French prosecutors said three coordinated teams, identically armed with Kalashnikov automatic rifles and explosive suicide vests, carried out the assaults using two cars registered in Belgium, where six people â€“ including three French nationals, one allegedly involved in the attacks â€“ were arrested on Saturday.
Announcing three days of national mourning and a national state of emergency, President FranÃ§ois Hollande called the coordinated assault â€“ â€œprepared, organised and planned overseas, with help from insideâ€ â€“ an â€œact of warâ€ that must be countered â€œmercilesslyâ€.
Mostefai, 29, was identified by his severed finger, which was found among the wreckage of the Bataclan concert hall, where three militants blew themselves up in the first suicide bombings on French soil late on Friday as security forces stormed the building.
At least 89 people died at the popular rock venue in the deadliest of six separate attacks, Europeâ€™s worst since the 1994 Madrid bombings, which left 191 people dead.