Monday's blast followed two other car bomb attacks the previous day.
At least 10 people were injured after suspected Eta fighters threw petrol bombs at a police station in Ondarroa in northeast Spain on Sunday before detonating a car bomb. That attack came only hours after a car bomb exploded in the regional capital of Vitoria, causing no casualties.
The attacks were carried out as the regional government prepared to lodge a complaint at the European Court of Human Rights over the Spanish government's refusal to let them hold a referendum on the Basque Country's future.
It also followed the supreme court's decision on Thursday to close down the Communist party of the Basque Lands on the grounds that it is linked to Batasuna, the banned political wing of Eta.
Earlier in the week, Basque Nationalist Action, another pro-independence party, and Gestoras Pro Amnistia, an advocacy group for jailed Eta members were also closed down.
The Basque Country, which has its own distinct language and culture, already has considerable autonomy over areas such as health and education.
More than 800 people have been killed by Eta in four decades of fighting to secure a homeland in northern Spain and southwestern France. The group abandoned a ceasefire in June 2007 after talks with the government collapsed.