On the day of the attack, ETA hijacked a car in Lasarte, threatening the owner, Claudio Rivera, with death if he informed the police before 16:00.

ETA had carried out a similar attack earlier in the year, killing six civil guards in an ambush at Ispaster.

The ETA unit, waiting near the bridge at the main road, launched grenades at the vehicles and subjected them to crossfire,[1] killing two of the guards and wounding three.

Another group of civil guards, who had arrived at the munitions factory to begin their shift, heard the shooting and rushed to assist their colleagues.

[1] Police searching the scene found shell casings which corresponded to those used in CETME and Remington guns.