People come and go at odd hours. The walls vibrate from jackhammers. Tenants pay rent in cash.
In San Diego's Otay Mesa industrial area, these are signs that warehouses may be housing cross-border tunnels used to smuggle huge amounts of drugs from Tijuana, Mexico.
After two major underground passages were discovered last month less than two blocks from one of nation's busiest border crossings for cargo, federal authorities are knocking on doors of warehouse owners and tenants to ask for help.
They hope to learn more about the 12,000 businesses that occupy Otay Mesa's nondescript warehouses and low-slung office parks. The streets hum with cargo trucks by day and fall silent on nights and weekends.
An Immigration and Customs Enforcement agent who visited two offices Thursday 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?
More 'eyes and ears'
"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.
The tunnels discovered last month ran about 2,000 feet and were equipped with rail car, lighting and ventilation systems. According to U.S. authorities, both were the work of Mexico's Sinaloa cartel, headed by that country's most-wanted drug lord, Joaquin "El Chapo" Guzman.
The discoveries produced some of the largest marijuana seizures in the United States.
A passage discovered Nov. 2 that ran between warehouses in Tijuana and San Diego resulted in seizures of 30 tons on both sides of the border. A tunnel found on Thanksgiving Day resulted that ran from a Tijuana home to two San Diego warehouses produced seizures of than 20 tons on both sides of the border
Outlets for the tunnel found last week were in bustling office parks, only 800 feet part. Looking back, neighbors said they missed clear warning signs.
One next-door neighbor remembers hearing voices through the walls at night and rarely seeing anyone during the day. The front door was spray-painted to prevent anyone from looking inside.
Mario Rodriguez, who worked next door to the tunnel's other outlet, said he never saw anyone come or go.
"We don't realize until it's too late," said Rodriguez, 46. "Look over there, across the street. I have no idea what they do."