At least three people were injured when a bomb exploded in one of the multi-storey car parks of Madrid's Barajas airport on Saturday, media reports said.
An anonymous caller had warned the emergency services 30 minutes ahead of the explosion, reports said.
Terminal 4 of the airport had been evacuated after the telephone warning. The injured were said to be three policemen checking a suspicious vehicle.
Though no one has claimed responsibility for the attack, there were speculations regarding the role of the Basque separatist terrorist group ETA.
ETA had announced a "lasting ceasefire" on March 22 but repeatedly threatened to resume its terrorist activities in recent weeks.