It would eventually continue with the GAL but it really began as early as 1933, just after the Second Spanish Republic was born. The self-proclaimed socialist and worker but also (and mainly) Spanish party sent armed thugs to kill Hermenegildo Alvariño in Barakaldo a day like today, November 19th, some 78 years ago.
His crime? To be a Basque Socialist instead of a Spanish whatever (no matter that his roots were in Galicia), to be a member of Basque Nationalist Action (EAE-ANV), the precursor of the modern Basque Nationalist Left.
Like tomorrow, there were elections to the Spanish Parliament that very day.
Source: Sare Antifaxsita[es].