Officials blamed Basque separatists for the death of a policeman who was killed Monday when a bomb exploded under his car in the northern city of Bilbao.
The explosion shattered windows and charred the facade of an apartment building.
It occurred moments after police officer Modesto Rico drove out of a garage on his way to work at a provincial court, a police spokesman said.
No one else was killed or injured.
Police said the attack bore the hallmarks of the separatist group ETA, which has stepped up its violent campaign for Basque independence with six killings so far this year.
“ETA’s killings only make us commit ourselves more deeply to the fight against violence and the murderers,” said Federico Trillo, speaker of Parliament.
ETA, which stands for Basque Homeland and Freedom in the Basque language, is believed responsible for nearly 800 deaths since it took up arms in 1968.