EFEBy Manuel Ayala Tijuana, Mexico

A modest one-story house in this northwestern Mexican border city served to conceal one end of a sophisticated narco-tunnel used to smuggle drugs to San Diego, California.

US authorities found the tunnel - equipped with reinforced walls, a rail system, ventilation and electricity - last Friday and announced their discovery on Monday.

The small Tijuana residence that marked the passage's starting point on the Mexican side of the border is situated in a working-class neighborhood.

The home is located just 800 meters (2,620 feet) from the Otay Mesa Port of Entry, one of three official border crossings in the San Diego-Tijuana metropolitan region, and 300 meters from a stretch of border wall that was reinforced two years ago as part of then-US President Donald Trump's plan to halt the northward flow of undocumented migrants.