On the night of 25 September 1985, José María Echaniz Maiztegui, Ignacio María Astiasuinzarra Pagola, José Sabino Echaide Ibarguren and José Agustín Irazustabarrena Uruzola were having a drink in the bar of the hotel Monbar in Bayonne (France), which was frequented by sympathisers of the Basque nationalist left and ETA members. Around 9.15 pm, when they were standing at the bar, two individuals got out of a car and entered the premises armed with machine guns. One of the terrorists stayed at the door to make sure nobody left the establishment while the other fired a number of rounds at the heads and chests of the customers.
Echaniz, Astiasuinzarra, Echaide and Irazustabarrena were killed and a fifth man, a French citizen, was seriously injured. Several witnesses chased the assailants on foot and managed to detain them until the French police arrived. The two men were mercenaries with links to the Marseilles criminal underworld who had been hired by GAL.
José María Echaniz Maiztegui, alias Potro, had fled to France in 1975 as a member of ETA militar. According to the Spanish police, he was the head of the ETA units in Vitoria.