PARIS — Thieves were able to penetrate a French military base in the south of France and escape with bomb-making matériel, despite heightened security in the wake of a terrorist attack at a gas and chemical factory last month.

In announcing the theft on Tuesday, the French Defense Ministry would not list the types or quantities of stolen matériel. But officials, who said the loss was discovered on Monday, did not dispute news reports that said at least 150 detonators and about 40 grenades, as well as an unknown amount of explosives, had been taken.

The ministry said two investigations had been opened: one into the theft itself and how it happened and the other into the security measures at similar sites throughout France, according to a statement.

Security was supposed to be tightened after an attack on an American-owned chemical plant near Lyon on June 26 by a man believed to have connections to Islamic extremists. The man, Yassine Salhi, 35, decapitated his boss before setting off an explosion, the authorities said, and he was arrested.