MEXICO CITY (AP) — Unidentified assailants have opened fire outside a city hall in the northern Mexico border state of Sonora, killing two men and a woman. The attack took place Thursday outside the city hall in the Gulf of California port of Guaymas. The state prosecutors office urged residents to avoid the downtown area, and police, Marines and National Guard units responded to the attack. Sonora has been the scene of bloody turf battles between several cartels, but it was unclear if Thursday’s shooting was related to that. On Wednesday, searchers found 14 clandestine graves in a town north of Guaymas.