An ETA commando unit using the code name Txikia (after the nom de guerre of ETA activist Eustakio Mendizabal, killed by the Guardia Civil in April 1973) rented a basement flat at Calle Claudio Coello 104, Madrid, on the route by which Blanco regularly went to mass at San Francisco de Borja church.[1]

Over five months, the unit dug a tunnel under the street – telling the landlord that they were student sculptors to hide their true purpose. The tunnel was packed with 80 kg (180 lb) of Goma-2 that had been stolen from a government depot.[citation needed]

On 20 December at 9:36 AM, a three-man ETA commando unit disguised as electricians detonated the explosives by command wire as Blanco's Dodge Dart passed.[2] The blast sent Blanco and his car 20 metres (66 ft) into the air and over the five-story church, landing on the second-floor terrace of the opposite side.[3] Blanco survived the blast but died at 10:15 AM in hospital.[2] His bodyguard and driver died shortly afterwards.[2] The "electricians" shouted to stunned passers-by that there had been a gas explosion, and then fled in the confusion. ETA claimed responsibility on 22 January 1974.

In a collective interview justifying the attack, the ETA bombers said:

The execution in itself had an order and some clear objectives. From the beginning of 1951 Carrero Blanco practically occupied the government headquarters in the regime. Carrero Blanco symbolized better than anyone else the figure of "pure Francoism" and without totally linking himself to any of the Francoist tendencies, he covertly attempted to push Opus Dei into power. A man without scruples conscientiously mounted his own State within the State: he created a network of informers within the Ministries, in the Army, in the Falange, and also in Opus Dei. His police managed to put themselves into all the Francoist apparatus. Thus he made himself the key element of the system and a fundamental piece of the oligarchy's political game. On the other hand, he came to be irreplaceable for his experience and capacity to manoeuvre and because nobody managed as he did to maintain the internal equilibrium of Francoism

— Julen Agirre, Operation Ogro: The Execution of Admiral Luis Carrero Blanco[4] The killing was not condemned and was, in some cases, even welcomed by the Spanish opposition in exile. According to Laura Desfor Edles, professor of sociology at California State University, Northridge, some analysts consider the assassination of Carrero Blanco to be the only thing the ETA have ever done to "further the cause of Spanish democracy".[5] However, former ETA member turned writer Jon Juaristi contended that ETA's goal with the killing was not democratization but a spiral of violence to fully destabilize Spain, heighten Franco's repression against Basque nationalism and force the average Basque citizen to support the lesser evil in the form of the ETA against Franco.[6]