French gendarmes secure a garage entrance, where an anti-terrorist raid was conducted four days ago, in Torcy, near Paris Oct. 10, 2012. (CHRISTIAN HARTMANN/REUTERS)

French police have broken up “an extremely dangerous terrorist cell” whose members possessed bombmaking materials and a handwritten list of Jewish organizations in the Paris region, the Paris prosecutor announced Wednesday.

The prosecutor, Francois Molins, said that as a result of the discoveries, 12 young men arrested Saturday would be retained for questioning for two more days under anti-terrorism legislation allowing six days of interrogation without charges in cases of a serious risk of “imminent” terrorist violence.

The 12 suspects, ranging in age from 18 to 25, included recent French converts to Islam as well as people born into Muslim families established in France, police said. Some of them were planning to go to Syria to fight alongside Salafist volunteers who have joined the uprising against the secular government of President Bashar al-Assad, Interior Minister Manuel Valls told the magazine Paris Match.

The French internal security agency, the Central Directorate for Domestic Intelligence, has intensified scrutiny of radicals among the estimated 5 million Muslims here since outrage exploded in Muslim countries last month over a YouTube video mocking the prophet Muhammad and an anti-Islam caricature in the weekly Paris satirical magazine Charlie Hebdo.

A man identified as a convert named Jeremie Louis-Sidney, 33, said to be the group’s leader, was shot and killed Saturday after he opened fire as police moved in to arrest him in the eastern city of Strasbourg.

Based on information obtained since the arrests over the weekend, police searched homes and storage facilities rented by the group in Cannes, Strasbourg and Paris. On Tuesday evening they found potassium nitrate, sulfur, pressure cookers and headlight bulbs at a storage facility in the Paris suburb of Torcy, elements that Molins said seemed intended for constructing homemade bombs.

“We are clearly and objectively confronted with an extremely dangerous terrorist cell,” he said in his televised statement.

Two of the suspects have been connected to a Sept. 19 incident in which a grenade was tossed at a kosher grocery store in the Paris suburbs, but it remains unclear whether they actually threw the grenade, Molins added. That attack, which resulted in one slight injury, generated a swift swelling of emotion among Jewish organizations and a pledge by President Francois Hollande that the government would go all out to discover and crack down on extremists living in French society.

Four people, including three children, were killed at a Jewish school in Toulouse in March when an Islamic radical, Mohammed Merah, went on a shooting rampage that claimed seven victims until Merah was killed by anti-terrorism forces after a long standoff at his rented apartment.