A French prosecutor says a group of people arrested last Saturday by police are believed to have been recruiting fighters to go to Syria as well as planning attacks in France.
Forensics experts said that bomb-making material discovered in a garage in the east of Paris was sufficient to make an explosive device similar to the one used in the last major bombing in the city in the 1990s, Francois Molins, a prosecutor, said on Wednesday.
Molins was speaking four days into an investigation in which police have quizzed 12 people who were rounded up in raids last Saturday. Another suspect was shot dead after he fired at police with a revolver.
The prosecutor said the police operation had dismantled what he called "a terrorist group that is probably the most dangerous [seen in France] since 1996".
Seven of the 12 were being placed under official inquiry on suspicion of terrorist activity, while five others taken into custody at the weekend were being released, he told a news conference.
Arms and ammunition found
Police moved in on the suspects after a late-September attack on a kosher food shop north of Paris.
Molins said that attack, using what he described as an M75 grenade of the kind made in the former Yugoslavia, was intended to kill, even if nobody died.
Follow-up surveillance of the suspects triggered last Saturday's police raids in the southern city of Cannes, in Strasbourg in eastern France and in Torcy, a suburb east of Paris where the police found the garage packed with bomb-making materials.
Along with a shotgun, rifle, 800 bullet rounds and cash, police found a large quantity of potassium nitrate, sulphur, five metres of cable, alarm clocks, headlamp bulbs and a pressure cooker that could serve as a bomb casing.
"It's exactly the same kind of device as was used in 1995 by GIA militants," Molins said, referring to the Algerian-based Groupe Islamique Arme, or Armed Islamic Group in English.
The 1995 attack was followed by another similar attack in 1996 in which two people were killed, taking the total death toll to 10 and the number of injured to about 200.
"I am not making comparisons but simply highlighting that in terms of danger and preparations - notably all the materials that were set to be part of the explosive device - we've not seen a similar case in French judicial police records since 1996," Molins said.
"Investigators rapidly unearthed very worrying details on the state of planning progress."
All seven suspects placed under official inquiry were suspected of taking part in "terrorist activities" and two of them were also suspected of playing a role in seeking volunteers to join missions abroad, including to Syria, said Molins.
One of the two suspected of recruiting fighters had made trips to Egypt and Tunisia, a former French colony, spending three months away from France with the man who was killed in the weekend raids, he said.
Analysts say armed groups could have many motives to stage attacks in France.
"The terror threat in France is extremely high because it comes in multiple forms," Roland Jacquard, president of the International Terrorism Observatory, said.
"You have Pakistan-Afghanistan connections that stem from old al-Qaeda networks, internal groups that have self-radicalised, al Qaeda in Islamic Magreb, and French intelligence services are also worried about violent terrorist reaction as a result of France's hard stance on Syria and Iran."