The investigation into the largest marijuana bust at a cross-border tunnel followed a familiar timeline. It began in May and ended in November.
The secret passage linking warehouses in San Diego and Tijuana - equipped with a hydraulic lift, electric rail carts and a wooden staircase - highlights an emerging seasonal trend. For three years, authorities have found sophisticated tunnels on the U.S.-Mexico border shortly before the winter holidays in what officials speculate is an attempt by drug smugglers to take advantage of Mexico's fall marijuana harvest.
Two weeks ago, authorities seized 17 tons of marijuana in connection with a tunnel linking warehouses in San Diego and Tijuana. Authorities began investigating that passage in June, according to court filings.
Tuesday's find netted more than 32 tons of marijuana - nearly 17 tons at a warehouse in San Diego's Otay Mesa area, about 11 tons inside a truck in the Los Angeles area and 4 tons in Mexico. According to the U.S. Drug Enforcement Administration, it ranks as the second-largest pot bust in U.S. history if the drugs found on the Mexican side of the tunnel are counted and the third-largest without the Mexican stash.
As U.S. authorities heighten enforcement on land, tunnels have become an increasingly common way to smuggle enormous loads of marijuana. More than 70 passages have been found on the border since October 2008, surpassing the number of discoveries in the previous six years
Raids last November on two tunnels linking San Diego and Tijuana netted a combined 52 tons of marijuana on both sides of the border. In early December 2009, authorities found an incomplete tunnel that stretched nearly 900 feet into San Diego from Tijuana, equipped with an elevator at the Mexican entrance.
Authorities say central Mexico's marijuana harvest in early October presents drug cartels with a familiar challenge for any farmer: how to quickly get products to consumers.
"It's a significant amount of inventory that the cartels need to move and they need to move it in the most expeditious and efficient way," said Derek Benner, U.S. Immigration and Customs Enforcement's special agent in charge of investigations in San Diego. "It's like any other business. You've got a pile of inventory that you need to get moving and generate profits."
William Sherman, the DEA's acting special agent in charge in San Diego, said drug traffickers also may go on a pre-Christmas smuggling push to give themselves a "little bit of hiatus" over the holidays to visit family in Mexico. DEA wiretaps tend to go quiet during the holidays, he said.
It's unclear whether cartels are building the tunnels in time for the winter holidays or if that's when authorities just happen to find them.
Some U.S. authorities are inclined to think the cartels are timing construction for the fall harvest, based on their belief that this year's two major finds in San Diego and one last year in San Diego were discovered shortly after they were completed. Heightened activity around building and operating the tunnels drew suspicion and exposed smugglers to getting caught.
It takes roughly six months to a year to build a tunnel, authorities say. Workers use shovels and pickaxes to slowly dig through the soil, sleeping in the warehouse until the job is done. Sometimes they use pneumatic tools.
The tunnel discovered Tuesday was about 40 feet deep, 4 feet wide and 4 feet high. It featured a wooden staircase at the U.S. entrance, located inside a large, white building with a long line of trucking docks.
The Mexican warehouse was on the same block as a federal police office and sits next to a runway at Tijuana's main airport. It featured a hydraulic lift at the tunnel entrance that dropped about 30 feet. Its carpeted floors were found littered with garbage and dirty linen. The kitchen was stocked with tortillas and oranges, with a window painted black.
Six men were charged in federal courts in Southern California with conspiracy to distribute marijuana. No arrests were made in Mexico.
U.S. authorities linked last November's find to Mexico's Sinaloa cartel, led by Joaquin "El Chapo" Guzman, that country's most-wanted drug lord. U.S. and Mexican authorities declined to link Tuesday's discovery to a specific cartel.