A bomb detonated at a rail station outside the Basque city of Bilbao early Monday, causing serious damage and igniting a fire, Basque police said. No one was injured.
The device exploded in Barakaldo, a town on the outskirts of Bilbao, Basque regional police said.
The blast occurred about half an hour after midnight and bore the hallmarks of the kind of attack waged by young supporters of the armed separatist group ETA. But a Basque police official said it was too early to say who was behind the explosion.
On Sunday police detained 18 fugitive members of an outlawed Basque youth group which the Supreme Court ruled last month is a terrorist organization because of its alleged links to ETA.
Spanish National Radio suggested the bomb blast on Monday might be in reprisal for these arrests.