TIJUANA, Mexico — Seven police officers were assassinated in about an hour's time in what authorities said Tuesday was a coordinated effort that followed months of relative calm in a border city stricken by drug-fueled violence. Three officers were injured. Municipal police detained three or four people after Monday night's attacks, said Jose Manuel Yepiz, spokesman for the Baja California state attorney general's office. He did not have additional details. After four officers were killed by gunfire outside a convenience store, police scanners hummed with "narcocorridos," or drug ballads. One voice threatened over the airwaves that 30 officers would be...