Speaking to state radio, Alberto Ruiz, Arrigorriaga's mayor, said: "The damage looks quite spectacular, there were some vehicles inside the building complex and the door has been blown off. But I don't think the damage is worth a lot."
The explosions came one day after judge Baltasar Garzon denied Inocencia Galparsoro, the mayoress of the Basque town of Mondragon, bail and sent her to prison, pending trial, for supporting the aims of ETA.
The separatist group's last fatal attack was believed to be on March 7 when, two days before a general election, a former councillor from Spain's governing Socialist party was shot dead in Mondragon.
Eta has killed more than 800 people in four decades of armed struggle for independence for the Basque Country from Spain.