US authorities seized a huge haul of marijuana after the discovery of a cross-border tunnel equipped with a railway cart.
The tunnel was 2,200 feet long and ran from the kitchen of a home in Tijuana, Mexico, to two warehouses in San Diego's Otay Mesa industrial district, said Mike Unzueta, head of investigations at US Immigration and Customs Enforcement in San Diego, California.
The Mexican entry featured a shaft lined with blocks and a rail system for drugs to be carried on a small cart, Mr Unzueta said. The tunnel had fluorescent lighting and a ventilation system.
Two men, both believed to be US citizens, were arrested in the US on Thursday on suspicion of conspiracy to smuggle and distribute drugs. The Mexican military raided a ranch in Mexico and made five arrests in connection with the tunnel.
US authorities followed a trailer from one of the warehouses to a Border Patrol checkpoint in Temecula, where they seized 27,600lbs of marijuana, Mr Unzueta said. The driver, whose name was not released, was arrested, along with another driver who went to a residence in suburban El Cajon that had 13,500 dollars in cash inside.
"That (trailer) was literally filled top to bottom, front to back," Mr Unzueta said. "There wasn't any room for anything else in that tractor-trailer but air."
An estimated three to four tons of marijuana was found in a "subterranean room" and elsewhere in the tunnel on the US side. Mexican authorities believe they seized another three to four tons in Mexico, potentially bringing the total to about 21 tons.
Earlier this month, agents made one of the largest marijuana seizures in the US when they confiscated 20 tons of marijuana they said was smuggled through another cross-border tunnel.
Mr Unzueta said both tunnels are believed to be the work of Mexico's Sinaloa cartel, headed by the country's most-wanted drug lord, Joaquin "El Chapo" Guzman.
"We think ultimately they are controlled by the same overall cartel but that the tunnels were being managed and run independently by different cells operating within the same organisation," Mr Unzueta said.