Suspected marijuana traffickers opened fire on Mexican state police near the farming town of San Fernando, killing four officers and a civilian, authorities said. The officers were ambushed by gunmen escorting a gasoline tanker truck believed to contain three tons of marijuana. Three officers who chased the fleeing suspects were wounded, officials said. A police spokesman said that the shootout left half the town's state police contingent either killed or wounded. Two suspects were arrested later in Reynosa, Mexico, across the border from McAllen, Tex. Texas authorities were searching for other members of the ring.