A bodyguard working for a local Socialist Party politician has died after being injured in a car bombing in Bilbao.
Basque separatist group Eta is suspected of carrying out the attack in the Basque city.
Spain has tightened security over fears Eta might attack on a major national holiday this Friday.
The holiday, known in Spanish as the Dia de la Hispanidad, commemorates Christopher Columbus's arrival in the New World and is Spain's armed forces day.
In Madrid, King Juan Carlos will preside over a military parade to mark the day.
Tensions are particularly high after Spanish police arrested 23 senior members of Batasuna, Eta's political wing, last week. Seventeen of them have been jailed on suspicion of belonging to the organisation.