Federal officials on Tuesday uncovered a sophisticated drug tunnel in Southern California that connected to a home across the border in Tijuana, Mexico, The Associated Press reported.

The tunnel, which was still under construction, featured lighting and a rail-car system. U.S. Border Patrol agents were alerted to the structure when they investigated a sinkhole close to the border fence, the news service detailed.

Immigration and Customs Enforcement (ICE) spokeswoman Lauren Mack told the AP that the officials had been patrolling west of the San Ysidro port of entry, one of the principal links between San Diego and Tijuana. The tunnel was about 220 yards in length, had a cart inside and its walls shored up, she added.

No drugs were found, and no arrests have been made, the ICE spokeswoman added.

U.S. officials notified Mexican authorities, who discovered the local entry to the tunnel at a residence in the Colonia Castillo neighborhood of Tijuana, the largest city in the Mexican state of Baja California, Hoy Los Angeles noted. Mexican soldiers and officials from the federal Attorney General's Office secured the site, the website added.

Drug cartels are increasingly constructing tunnels to smuggle narcotics into the United States, according to the AP. More than 75 such structures have been discovered along the U.S.-Mexico border since 2008, the news service said.

Hours before the San Ysidro tunnel was reported, U.S. Border Patrol agents had already unveiled what Hoy Los Angeles called a "sophisticated passageway" between Mexicali, the capital of Baja California, and the adjacent Californian city of Calexico.

That structure was discovered as American officials followed four individuals who they said tried to smuggle drugs along the All-American Canal, an aqueduct in southeastern California; one of the suspects was arrested by El Centro-based border guards, while the remaining three were able to escape U.S. jurisdiction by returning onto Mexican territory.

When they inspected the Calexico tunnel, authorities found a diving kit along with packages containing 69 pounds of methamphetamines, an extremely addictive stimulant drug, Hoy Los Angeles detailed.