SAN DIEGO -- Mexican authorities discovered a tunnel under construction beneath a Tijuana house a block from the U.S.-Mexico border, near the Otay Mesa border crossing, 10News reported.

The 30-foot-wide tunnel is the fourth to be found near the border in the last year.

The hole extends 59 feet underground and has a ladder leading to the surface.

"Nothing has been found at this time that indicates a tunnel leads into the United States," Vince Bond, spokesman for the U.S. Customs Service, told the San Diego Union-Tribune.

Bond said that the U.S. agencies were not involved in the investigation because the tunnel is in Mexican territory.

A Mexican official, who asked to remain anonymous, told the newspaper that U.S. drug enforcement agents arrived Tuesday to inspect the site.

"It seems that this is a tunnel under construction," the official told the newspaper.

U.S. and Mexican officials would not speculate on whether the tunnel was built by smugglers. But Peter Nunez, a former U.S. attorney in San Diego, suggested that well-organized drug traffickers paid for its construction.

"Small-time operators can't handle building a tunnel," Nunez told the Union-Tribune. "It requires too much skill and is too expensive."

The home was being guarded by Tijuana police and Mexican federal agents armed with machine guns.