The scale of the operation pointed to the work of a major Mexican drug cartel, authorities said, and comes two weeks after Mexican authorities discovered a record 134 tons of marijuana in an industrial area near Tijuana.
Officials don't know if there is a connection between the two events, but called this week's discovery another significant blow against organized crime groups.
Authorities estimated the drugs' worth at more than $20 million.
"I can promise you there are some very unhappy people in the cartel," said John Morton, director of U.S. Immigration and Customs Enforcement, which leads the multi-agency San Diego Tunnel Task Force.
The investigation was triggered Tuesday afternoon when task force agents patrolling the light industrial area near the Otay Mesa Port of Entry followed a tractor trailer that was acting suspiciously, Morton said.
The truck was stopped at a U.S. Border Patrol checkpoint in Temecula, where agents found about 10 tons of marijuana stuffed in the 53-foot trailer. Two people in the truck, a husband and wife whose identities were not disclosed, were arrested. Agents later raided the warehouse from which the vehicle had departed and discovered another 10 to 15 tons of dope, the bales wrapped in cellophane and on pallets, as if ready for shipping.
Inside a storage closet they found a 3-foot by 4-foot tunnel opening cut into the subfloor. The passageway, featuring lighting, ventilation and a rail system, descends about 20 feet below ground and goes under Via de la Amistadand another warehouse before continuing under the border fence into Mexico.
Mexican soldiers on Wednesday raided a warehouse south of the border and found about 4 tons of marijuana, according to the Mexican military. U.S. authorities wouldn't disclose how the Mexican entry point was found, but agents in past cases have located openings by traversing the entire length of the tunnel.
In this case, such a crossing would have been extremely difficult. Officials said the tunnel was only 4 feet high by 3 feet wide. The toil and financing required for such an undertaking was further evidence that above-ground enforcement efforts are forcing cartels to extreme measures to get their drugs across the border, officials said.
The tunnel is one of the few unearthed in recent years that was fully operational. Authorities estimate operators had been smuggling drugs through it for less than one month. About 75 tunnels along the U.S.-Mexico border have been unearthed in the last four years, most of them in various states of construction.
The operation was also notable because of the quick response by Mexican authorities. In past cases, their less-than-prompt actions have allowed tunnel operators in Mexico to clear out the drugs. This time they reacted immediately, which authorities said reflects the much-improved levels of cooperation from security forces in Tijuana, which are led by army Gen. Alfonso Duarte Mugica.
"The Mexicans moved as quickly as we did," Morton said. "It was an example of the coordination needed to be successful."
There were no arrests in either warehouse, and it's unclear who owned the drugs or whether the owners of the facilities were involved. The investigation is ongoing, authorities said.
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