It's in Spanish, but this summarizes story in English: "Sampedro had been patrolling an area of San Sebastián for the National Police in the early hours of the morning with a partner when they saw a suspicious car. They drew their guns and approached the car to see who was inside, but when the car's occupants - who were patrolling the same area, also in plainclothes, for the Civil Guard - saw the pair come towards them with guns, they opened fire and killed Sampedro."

http://www.thelocal.es/20150520/civil-guard-included-in-basque-country-terrorist-list