TRONCONES, Mexico — Gunmen believed to be drug traffickers shot an American consulate worker and her husband to death over the weekend in the violence-racked border town of Ciudad Juárez, and killed the husband of another consular employee and wounded his two young children, the authorities said Sunday. President Obama expressed outrage at the “brutal murders” and in a statement from the White House vowed to “work tirelessly” with Mexican law enforcement officials to bring the killers to justice.