Speaking at a summit in El Salvador, Zapatero and Carlos also spoke out against the group allegedly responsible for the attack, El Mundo newspaper reported Friday.
The explosion at the University of Navarra in Pamplona Thursday has been blamed on the Basque separatist group ETA.
ETA is blamed for more than 800 killings since 1968. The European Union and the United States list ETA as a terrorist group.
Celebrity Families of 2014 [PHOTOS]