The Basque separatist group, ETA, was responsible for today's explosion near Zaragoza Airport, which is used by Ryanair.
A warning telephone call in the name of ETA was made to a local newspaper before the explosion.
A Spanish government spokesman said the airport was evacuated before the blast and no-one was injured.
Flights were suspended after police sealed off the zone, and there were also reports that three rocket launchers and at least two mortars had been found.
A Ryanair plane en route from London was diverted to a nearby military airport after the explosion. It later returned to London after unloading its passengers, media reports said.
