A car bomb exploded Thursday in the parking lot of the University of Navarre in northern Spain, injuring 21 and setting many vehicles ablaze.
Spanish officials blamed the Basque separatist organization ETA for the attack.
Spanish Interior Minister Alfredo Perez Rubalcaba said a telephone call in the name of ETA an hour before the blast had warned of a car bomb left in the university, but "the one who put the bomb did not warn correctly or wanted to mislead," making it impossible to deactivate the device.
Some 1,000 students were in the nearby buildings when the 70-80 kg of explosives went off, and it could have led to a terrible tragedy, Perez added.
The University of Navarre is in the northern Spanish city of Pamplona, close to Basque country, a repeated target of bomb attacks by ETA separatists.
The bomb attack came just two days after Spanish police arrested four suspected ETA guerrillas, but Perez did not make any declaration on the possible relation between the two events.
ETA are said to have killed more than 800 people in four decades of armed struggle for independence of ancient Basque territories in Spain and France, including Navarre.
University officials said there were new bomb warnings after the explosion, causing an evacuation of students in all buildings of the university.