The 69-year-old admiral, his bodyguard and a driver died on December 20th, 1973 after Basque separatist group ETA detonated a bomb in a tunnel dug under the street in Madrid’s upscale Salamanca neighbourhood as his car passed.

The massive explosion opened a huge crater and sent the vehicle hurtling over 20 metres (65 feet) into the air before landing in the patio of a convent.

The first police officers who arrived at the scene thought it had been a gas explosion, not an assassination, since they did not immediately see the prime minister’s vehicle.

Carrero Blanco, a longtime advisor to Franco who had been prime minister for just six months, had just left Mass and was on his way to his office when he was killed.

It was the first time that a government minister had been killed in such violent circumstances since Franco came to power after the end of Spain’s 1936-39 Civil War, and it revealed the vulnerability of his regime, which
responded to the assassination by stepping up its repression.

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A frail-looking Franco was seen weeping in public for the first time at Carrero Blanco’s funeral. He died less than two years later, paving the way for Spain’s transition to democracy.

The dictator was “never the same, he got into a physical and psychological slump”, Spain’s foreign minister at the time, Laureano López Rodó, said in a Spanish public television documentary about Carrero Blanco’s assassination.

The impact of Carrero Blanco’s death on Spanish history is still hotly debated in several books published to coincide with the 50th anniversary of his assassination.

Carrero Blanco had served as deputy prime minister for five years before he was appointed premier in June 1973, and had played a key role in stifling opposition and propping up Franco’s hard-line regime.

Some historians believe his assassination eliminated the possibility the dictatorship could continue after Franco’s death with him at the helm, while others argue opposition to the dictatorship was already on the rise and the restoration of democracy was inevitable even if he had not been killed.

The meticulously planned assassination also catapulted ETA to the international limelight. Founded in 1959 at the height of Franco’s dictatorship, which repressed Basque culture and language, ETA is accused of
killing more than 850 people in its fight for an independent Basque homeland in northern Spain and southwest France.

The group announced a permanent ceasefire in 2011 and formally disbanded in 2018.