French police said they have discovered more than 300 kilograms of explosives in a garage in the southeastern city of Grenoble.
French news reports quoted an unidentified official as saying the cache discovered Sunday contains ammonium nitrate and other bomb-making material and equipment. They suspect the materials belong to the militant Basque separatist group ETA.
The owner of the garage became suspicious and alerted the police after the people who rented the garage were not seen for months.
ETA is responsible for at least 820 deaths in its 40-year long campaign for an independent Basque homeland in northern Spain and southwestern France. The group is based in Spain, but has members in France.
The Spanish government says the organization has weakened after the arrest of top ETA leaders in recent years. But smaller groups still carry out sporadic attacks.
Some information for this report was provided by AFP and Reuters.