The Spanish Interior Ministry said he was "on the most wanted list of ETA members" for giving support to one of the group's most active units, which operated in the San Sebastian region.
Sabin Mendizabal Plazaola disappeared in 2009 after a Spanish court sentenced him to prison for throwing an explosive device at the headquarters of Spain's ruling Popular Party in the Basque town of Ermua.
He was also was accused of throwing Molotov cocktails at a business owned by the Popular Party mayor of the city of Vitoria in 2005.
ETA, which means Basque Homeland and Freedom in Basque, is blamed for the deaths of 829 people in a four-decade campaign of shootings and bombings aimed at creating an independent Basque homeland in northern Spain and southern France.
