A masked assailant believed to be a Basque separatist shot and killed a police officer Saturday in this northern city, police said.

The gunman got out of a car about 500 feet from a police station and shot Cristobal Diaz Garcia in the chest as he stood guard just after 10 a.m., police said.No group claimed responsibility, but police said it appeared to be the work of the Basque separatist group ETA. Authorities said a witness told police the sniper and another armed and masked man fled in a car driven by a woman. Police found the car abandoned about five miles from the scene, they said.

Garcia, 36, was taken to a hospital and died in surgery, police said.