LOS ANGELES  Federal investigators discovered a sophisticated cross-border tunnel Thursday in an industrial part of San Diego. The half-mile tunnel was the second found this month equipped with rail tracks and carts to funnel drugs to and from Tijuana, Mexico.

After getting a tip about drug activity at a warehouse in Otay Mesa, a thicket of warehouses and truck repair shops that hugs the Mexican border, agents with the San Diego Tunnel Task Force arrested three men there and discovered the tunnel. United States and Mexican authorities have seized more than 20 tons of marijuana since Thursday.

Mexican military investigators later detained five men in Tijuana and uncovered an entrance to the tunnel beneath the kitchen floor of a house.