Alfredo Díez Marcos and another five other civil guards (Antonio Marín Gamero, José Gómez Martiñán, José Gómez Trillo, José Martínez Pérez and Victorino Villamor) were on duty in a convoy of two Land Rovers escorting a vehicle belonging to the arms factory Esperanza y Cía. They left the factory in Marquina at around 7.30 am on Friday 1 February.
The convoy was being watched by a number of terrorists who overtook it and teamed up with other ETA members at Kilometre 53, in the environs of Ispáster. At around 8.15 am, the convoy reached a winding stretch of the road where it had to reduce speed. It was there that it was caught in the mortal ambush. The two Civil Guard Land Rovers were attacked with assault rifles, machine guns and hand grenades. Over a hundred rounds were fired in all. The officers inside the Land Rovers were hit by the terrorist gunfire and either badly wounded or killed in the vehicles.
After the attack, several terrorists approached the vehicles and tossed a hand grenade into the first of the Land Rovers to ensure that none of the civil guards would survive. They then tried to do the same with the second vehicle. However, the grenade exploded before the killers had time to get away. Two of them (Gregorio Olabarría Bengoa and Javier Gorrotxategui Argote) were seriously wounded. Although the other members of the ETA unit took their wounded colleagues to a nearby bar, where they asked the owner to call a doctor, Olabarria died before he arrived. Gorrotxategui was also killed. The other terrorists left his body at the gate to the municipal cemetery of Ermua, wrapped in a Basque flag.
The killing of the six civil guards sparked outrage in Spanish society, prompting Prime Minister Adolfo Suárez to create a Special Security Delegation for the Basque Country and Navarre.