LA UNIÓN, Mexico — Gunmen believed to be linked to drug traffickers shot a pregnant American consulate worker and her husband to death in the violence-racked border town of Ciudad Juárez over the weekend, leaving their baby wailing in the back seat of their car, the authorities said Sunday. The gunmen also killed the husband of another consular employee and wounded his two young children. The shootings took place minutes apart and appeared to be the first deadly attacks on American officials and their families by Mexico’s powerful drug organizations, provoking an angry reaction from the White House. They came...