MEXICO CITY - The red tide of narco killings continued in the resort city of Acapulco, Mexico, this week, with the severed heads of two men discovered Friday, officials said. A day earlier, another head was found on the stairs of City Hall. All bore messages directed at the Nuevo Laredo-based Zetas, the enforcement arm of the Gulf cartel that operates along the Mexico-Texas border. The Zetas are in a vicious turf war with the Sinaloa cartel, named for the central Mexican state where it is based. The message found with the first decapitated head, discovered in a black plastic...