The attack occurred on a Friday morning. ETA had observed similar convoys from the factory and planned their ambush based on information that they had gathered. The convoy consisted of four vehicles, the first carrying technicians, the third mortar devices produced by the factory and the second and fourth vehicles carrying three civil guards each. The vehicle left the factory at 0730 and at approximately 0815, had to slow down at a part of the road which consisted of numerous bends. The ETA members had chosen a spot near a small hill, where they were hidden from view by dense vegetation. After allowing the vehicle carrying the factory workers to pass, ETA launched a grenade at the second vehicle, causing it to be stuck in a layby and then attacked the civil guard's vehicles with assault ri