(Adds details, background) MADRID, April 17 (Reuters) - A bomb exploded outside an office of the ruling Socialist Party in the Basque city of Bilbao on Thursday, wounding three police officers who had cordoned off the area after a warning from ETA, police said. The bomb, which was hidden in a bag, exploded at 6 a.m. (0400 GMT), half an hour after an anonymous caller told police where it had been placed and claimed responsibility for Basque separatists ETA, a police spokeswoman said. Two of the policemen suffered acoustic trauma, she said. The extent of the other's injuries was unknown. ETA has killed more than 800 people in four decades of armed struggle for independence for the Basque Country from Spain. The guerrilla group's last fatal attack was on March 7 when, two days before a general election, it shot dead a former Socialist Party councillor in the Basque town of Mondragon. It admitted responsibility for the killing on April 1. (Reporting by Jason Webb; Editing by Elizabeth Piper)
