Police in Spain have discovered a container with 100 kilograms of explosives in the northern Basque region.
The discovery came five days after a car bomb, blamed on Basque separatists, destroyed a parking garage at Madrid airport, killing at least one person and injuring about 20.
Spanish Prime Minister Jose Luis Rodriguez Zapatero visited the blast site Thursday and vowed again to stamp out violence. He has already called off peace talks with the Basque separatist group ETA.
ETA had declared a cease fire last March in its nearly 40-year campaign of violence for a Basque homeland in northern Spain and southern France.
A leader of the outlawed political wing of ETA told Basque radio Wednesday that the Madrid bombing caught his movement by surprise.
Also Thursday, Spanish police say they arrested a sixth suspect in the 2004 terrorist bombing of four commuter trains in Madrid, killing 191 people.
Police identify the suspect as Taha Seghrouchni. They arrested him in the town of Lleida. He and five other suspects arrested Wednesday are accused of helping key Moroccan suspects in the bombings escape.
Some information for this report was provided by AFP and AP.