"We are clearly and objectively facing an extremely dangerous terrorist cell," Paris prosecutor François Molins said Wednesday.
"Components useful for bomb-making" were found overnight during police searches of buildings in the eastern Paris suburb of Torcy, where two of the suspects were detained on Saturday as part of a nationwide round-up, he said.
Bags of potassium nitrate, sulphur, saltpetre, pressure cookers and headlight bulbs, "all products or instruments useful in the making of what we call improvised explosives", were found along with a shotgun and a handgun and a list of Jewish organisations in the Paris area.
The 12 suspects rounded up on Saturday were to be kept for at least one extra day over the usual four-day maximum, due to a 2006 change to the criminal code that allows detention for up to six days if there is a "serious risk of an imminent terrorist attack".
The group’s suspected leader, 33-year-od Jérémie Louis-Sidney, a convicted drug dealer who converted to Islam in prison, was shot dead on Saturday after opening fire on police trying to arrest him at his home in Strasbourg.
Forensic examinations showed that he had handled the grenade used in an attack on a kosher grocery on 19 September but Molins said Wednesday that he may not have thrown it and that two men believed to have been directly involved were probably still at large.
All of the suspects, one of whom is reported to be Tunisian, are refusing to cooperate with interrogation by anti-terrorist officers, sources told the AFP news agency.
Wiretaps and physical surveillance lasting three weeks showed that they were all “very active, mobile and extremely careful about their movements”, the source said.