The EiTB headquarters and the surrounding area were evacuated just minutes before the blast after a person claiming to represent Eta called the local fire service warning of an imminent explosion at the site.
Police spotted a suspicious vehicle near the building, and its owner was later found tied up in local mountains.
Media workers have frequently been the targets of Eta, which is blamed for the deaths of 825 people in its 40-year campaign for an independent Basque homeland comprising parts of northern Spain and southwestern France.
Police operations
Eta officially called off a 15-month-old ceasefire in June last year, saying it had grown tired of a lack of concessions on the part of the socialist government of Jose Luis Rodriguez Zapatero, Spain's prime minister, in their tentative peace talks.
That truce had effectively ended when Eta bombed a car park at Madrid airport on December 30, 2006, killing two Ecuadoran men who were sleeping in their cars.
Eta is blamed for the deaths of four people this year, including a municipal councillor killed just days before general elections in March and a Basque businessman who was gunned down in the street early this month.
Police operations in Spain and France earlier this year led to the arrest of Javier Lopez Pena, the group's presumed overall leader, in the French city of Bordeaux in an operation Spanish officials at the time claimed had "decapitated" the organisation.
In November, French police arrested Miguel de Garikoitz Aspiazu Rubina, known as 'Txeroki', who is suspected of being the head of Eta's military operations.
Earlier this month, Aitzol Iriondo Yarza, Txeroki's suspected successor, was detained in southwestern France.
France and Spain have stepped up co-operation to crack down on Eta since a special accord was signed in January allowing Spanish agents to operate in southwestern France.