Transportation officials received a bomb threat before the blast, which left two police officers and another individual with minor wounds, CNN reported.

The warning, which targeted a construction site in the northern port city, allowed police time to cordon off the building and keep area residents inside their homes.

The attack in Spain's Basque region came two days after the separatist group ETA said it would continue its battle for Basque independence.

The ETA has stepped up its attacks since the arrest of its suspected leader in France, CNN said. To date, ETA has been blamed for killing more than 800 people in the last four decades.