Workers at warehouses just feet from the U.S-Mexico border would come and go at odd hours. Few, if any, knew what their trucks carried.
Few knew, too, that the buildings sat atop the openings for two of the longest, most sophisticated tunnels ever built to smuggle drugs from Mexico to the United States.
"Look over there across the street, I have no idea what they do," said neighbor Mario Rodriguez, 46, pointing to a large warehouse with no trucks outside.
Now, U.S. authorities are stepping up scrutiny of warehouses near San Diego's Otay Mesa border crossing after the twin discoveries last month resulted in some of the largest marijuana seizures in U.S. history.
Federal agents are trying to enlist warehouse managers and workers to help recognize the telltale signs of tunneling. Watch for construction equipment and piles of dirt, they say. Listen for the sound of jackhammers. And recognize the scent of unburned marijuana.
For owners, they should think twice when tenants pay rent in cash, the agents say. Or if tenants offer little information about their businesses. And if workers come and go at odd hours.
"There is obviously a major problem," said Mike Unzueta, head of Immigration and Customs Enforcement investigations in San Diego. "We need to kick it up."
The San Diego Tunnel Task Force agents began visiting owners and tenants last week to gather intelligence on some of the 12,000 businesses in the industrial complex near one of the nation's busiest border crossings for trucks.
The agents peppered managers with questions before asking to look around: What line of work are you in? Who is your landlord? How many neighboring suites are leased?
"We're trying to get as many eyes and ears in the community as we can," Jonathon White, the Drug Enforcement Administration's San Ysidro agent in charge, reassured one manager, a customs broker, during a visit Thursday.
More than 125 cross-border tunnels have been found along the U.S.-Mexico border since the 1990s, though most are crude and incomplete. In Nogales, Ariz., smugglers tap into vast underground drainage canals. California is popular because its clay-like soil is easy to dig with shovels.
Otay Mesa has the added draw that there are plenty of buildings on both sides of the border to conceal trucks getting loaded with dope. Its streets hum with semitrailers by day and fall silent on nights and weekends.
"You can fit right in with your tractor-trailer," said Tim Durst, head of the tunnel task force, whose members have traveled to Israel and Egypt to work with governments there on how to detect tunnels in the Gaza Strip.
In 2006, ICE agents found a tunnel that ran the length of eight football fields in another Otay Mesa warehouse, the longest-ever cross-border passage. There was a reception room with desks and computers, but the warehouse was empty.
That discovery led to a federal law that can punish warehouse owners with 10 years in prison if they knowingly house a tunnel. None have been charged.
Durst said tunnel excavators advance about 10 feet a day during construction and sleep in warehouses. Some workers, he said, are killed when the job is done to prevent them from spreading word on the location.
The recent tunnels are remarkably sophisticated, with railcars, as well as lighting and ventilation systems.
A tunnel seizure that netted 30 tons of marijuana on both sides of the border Nov. 2 ended in an empty Otay Mesa warehouse that authorities found with a pile of dirty pizza boxes inside.
A 2,200-foot passage found on Thanksgiving Day with outlets in two Otay Mesa warehouses netted 20 tons of pot on both sides of the border.
At one of the warehouses there was a spray-painted door, an inflatable children's pool, presumably used to mix concrete, and a floor door that opened to the tunnel with the press of a button.
The tunnel entry at the second warehouse led down a narrow passage of about 20 steep stairs carved into the ground. It led to a long corridor with plywood neatly laid on the floor and lined with a rail track.
ICE had been watching one of the warehouses after San Bernardino County sheriff's deputies pulled over a truck driver in June outside Los Angeles, with 19 tons of marijuana, 2,700 pounds of cocaine and 67 pounds of methamphetamine.
Around the crack of dawn on Thanksgiving, agents followed a truck to a Border Patrol checkpoint and found that it was packed with pot.
U.S. authorities say both recent tunnels are the work of Mexico's Sinaloa cartel, headed by that country's most-wanted drug lord, Joaquin "Chapo" Guzman.
"As enforcement grows tighter above ground, the cartels are having to these huge extremes to get their drugs into the country," Unzueta said.