French police have discovered bomb-making material in a car park linked to the weekend arrests of a dozen terror suspects, the state prosecutor said.
Francois Molins said the material was found in a suburb east of Paris.
Twelve people were arrested last weekend over the firebombing of a kosher grocery store north of the capital last month. Mr Molins said the detention of the suspects will be extended for 24 hours.
Mr Molins said police investigators recovered items including potassium, sulphur, saltpetre (potassium nitrate) and a pressure cooker as well as two weapons in the car park in the town of Torcy, east of Paris.
They were led there by information from interrogations carried out since the arrest of the suspects in cities across France at the weekend, he said.
"We can say that we are clearly and objectively facing an extremely dangerous terrorist cell," Mr Molins said, adding that authorities are extending the detention of the suspects for 24 hours beyond the normal limit of 96 hours because of the seriousness of the case.
He added that it was only the second time since 2006 that the law allowing the extension of custody in such circumstances had been invoked.
The weekend arrests - during which one suspect was killed by police who returned fire after he shot at them - were put in motion by evidence gathered following the shop firebomb attack.
Police found weapons, cash and a list of Paris-area Israeli associations during the raids. It remains uncertain whether those suspected of directly carrying out the grocery store attack have actually been apprehended.
Mr Molins said at the time that all the arrested suspects were French and recent converts to Islam. Four of the men involved in the raid had written wills. One was carrying a loaded gun when arrested.