13 September 1974,
ETA broke a bomb in the Rolando cafeteria in Madrid and killed many civilians. In front of the cafeteria was the headquarters of the police and, according to the main thesis, it was a failed attack to murder the police.
It resulted in 13 deaths and 70 injuries, the majority of citizens outside the repression and in the separation between the military and politico-military branch of ETA, an attack that caused enormous tensions within it.
ETA initially denied her authorship and assumed it in the last release of 2018, while apologizing to civilian victims.
Source:
Emilio López de Adam, Armed Struggle in the Basque Country (1967-2011). First volume 1967-1980 (May).