French police found bomb-making materials during an investigation into radical Islamist suspects arrested at the weekend, a prosecutor says.
Police discovered chemicals and guns at a garage in the Paris suburb of Torcy, Paris prosecutor Francois Molins said.
He said the detention of 12 suspects would be extended by 24 hours.
On Saturday, police carried out raids in several cities and shot dead a man said to be linked to a grenade attack against a Jewish shop last month.
The materials found in Torcy overnight included potassium nitrate, sulphur, headlight bulbs, and pressure cookers, Mr Molins told journalists on Wednesday.
"These are all products used to make what we call improvised explosives," he said. A handgun and a shotgun were also found.
"We are clearly confronted with an extremely dangerous terror network."
Those arrested at the weekend were targeted as part of an investigation into an attack on 19 September on a kosher grocery in the Paris suburb of Sarcelles, in which one person was injured.
Mr Molins said that although two of the suspected organisers of the attack had been caught, it was not clear whether those who threw the grenade had been found.
The alleged leader of the group, 33-year-old Jeremie Louis-Sidney, was shot dead at his home in Strasbourg on Saturday in an exchange of fire with the police.
An ex-prisoner, who served time for drug-trafficking, he had recently converted to Islam.
Videos recovered from his apartment show him rapping about the 11 September 2001 attacks on the US, and boasting that it was just the start.
Officials have indicated that those arrested were French converts. Several were said to have been carrying copies of their wills.
After Saturday's raids, blank bullets were fired outside a Paris synagogue.
French President Francois Hollande said there would be "total mobilisation of the state to fight all terrorist threats".