Police have discovered bomb-making materials in a car park near Paris during an investigation into an "extremely dangerous terrorist cell" linked to an attack on a Jewish shop.
Some of the 12 suspected cell members arrested over the weekend appeared to have plans to go to Syria to fight in its civil war.
Paris prosecutor Francois Molins said the car park discovery in Torcy, east of the capital, led to employing a rarely-used legal clause to allow extended questioning of the suspects by a day - and possibly two.
France has been on high alert for possible terror attacks by radical Islamists after a Frenchman who claimed links to al Qaida shot and killed three Jewish children, a rabbi and three paratroopers in southern France in March. One counter-terrorism official said authorities were working to try to verify a possible link to Syria.
The case amounts to one of the biggest break-ups of a suspected Islamic terror cell in years in France, and the international link revived memories of the height of the Iraq war in the mid-2000s - when anti-terror groups dismantled a string of feeder cells that sent or plotted to send fighters to join the combat against Western forces there.
French authorities have been particularly concerned that the young militants might gain important skills in combat, bomb-making and other terrorism tactics abroad - then later return home to carry out attacks.
Investigators were led to the car park by information collected during questioning of the suspects, who were arrested in weekend raids across France in a probe of a firebombing of the grocery north of Paris last month. Two organisers of a September 19 grenade attack in Sarcelles are in thought to be among them, but the two people who actually carried it out could still be at large.
In Torcy, investigators found bags of potassium nitrate, sulphur, saltpetre, headlight bulbs and a pressure cooker - "all products or instruments useful to make what are called 'improvised explosive devices,'" Mr Molins said. "We can say that we are clearly and objectively facing an extremely dangerous terrorist cell," he said.
French president Francois Hollande said: "Let's let the investigations play out, because we haven't brought everything to light" in the case.
On Saturday, police shot the suspected leader of the cell, Jeremie Louis-Sidney, after he opened fire on them during a raid in Strasbourg. Police found weapons, cash and a list of Paris-area Israeli associations after raids in Strasbourg, near the Riviera resort of Cannes, and in the Paris area - including Torcy.