The City Hall of Villa Union is riddled with bullet holes after a gun battle between Mexican security forces and suspected cartel gunmen on Saturday. At least 21 people were killed, four of them police officers when an armed group in a convoy of trucks stormed the town 40 miles southwest of Eagle Pass, Texas. (Gerardo Sanchez/AP) MEXICO CITY — Gunmen attacked a town hall in northern Mexico, authorities said Sunday, triggering a series of clashes with security forces that left 21 dead — most of them suspected members of an organized crime group.The attack started at around noon on Saturday, when men...