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ETA, a Basque separatist organization, was responsible for the assassination of Luis Carrero Blanco, the Prime Minister of Spain from 1973 to 1976. The assassination took place on December 20, 1973, in Madrid. ETA used a car bomb to kill Carrero Blanco as his vehicle passed over a tunnel. The assassination was a major event in Spanish history, and it had a significant impact on the country's political landscape.