An anonymous caller phoned the Municipal Police of Vitoria warning that a bomb had been planted on the rail line. A Civil Guard bomb disposal squad, escorted by a patrol from the Rural Action Group and another unit from the Civil Guard station in Alegría, went to the scene.
The killers had placed a decoy device on the rails and hidden a second bomb on a path surrounded by bushes and at some distance from the decoy. A passenger train was halted on the track, while the specialists determined whether or not the bomb was real. As they were approaching the fake device, inspecting the track as they went, one of the officers tripped over a thin piece of fishing line hidden in the undergrowth. This triggered the detonator and set off the booby trap bomb on the embankment, containing around eight kilograms of Goma-2.
Eight civil guards were caught in the powerful explosion. Victoriano Collado Arribas, Agustín Pascual Jove and José Luis Veiga Pérez were killed instantly. Another five officers were injured, one of them —Manuel Gallardo— seriously.