On the morning of 14 July 1987, ETA attacked a four-vehicle convoy of Rural Anti-terrorist Groups in which fourteen civil guards were travelling on a road in the town of Oñate, Gipuzkoa. The ETA members had installed an explosive device next to the road, behind a wall, embedded in the ground.
The terrorists detonated the bomb as the convoy passed. The SUV was caught in the blast and officers Pedro Galnares Barreda and Antonio López Martínez Colmenero were killed instantly. Several Civil Guard officers suffered injuries of varying severity: Andrés Castillejo Martín, José Martínez Rodríguez, Agustín Álvarez Belmonte, Francisco Montes Urbano, Benedicto Martín, Pedro Delicado Navarro, Antonio Grande Lozano, Manuel Tejeiro Méndez, Antonio Martínez Hernández, José Rodríguez Chacopino, José Antonio González García and Juan Barreal Fernández.