Most trucks enrolled in the program pause at the border for just 20 seconds before entering the United States. And nine out of 10 do so without anyone looking at their cargo. But among the small fraction of trucks that are inspected, authorities have found multiple loads of contraband, the most common of which is marijuana.

Some experts now question whether the program makes sense at a time when drug traffickers are willing to do almost anything to smuggle their shipments into the United States.

The trusted-shipper system "just tells the bad guys who to target," said Dave McIntyre, former director of the Integrative Center for Homeland Security at Texas A&M University.

The program works like this: Participating companies agree to adopt certain security measures in exchange for fast entry into this country. They are required to put their employees through background checks, fence in their facilities, and track their trucks.

They also are asked to work with subcontractors who also have been certified under the program, which is run by U.S. Customs and Border Protection.

The government keeps the participant list secret, citing national security and trade secrets. But some of the 9,500 companies in the system advertise their membership to drum up business, making them targets for smugglers, who can then threaten drivers or offer them bribes.

More than half of all U.S. imports come from companies in the program, which is called the Customs-Trade Partnership Against Terrorism, or C-TPAT. Mexican trucking companies make up only 6 percent of global membership in the system but account for half its 71 security violations the last two years.

Mexican trucking firms face higher scrutiny than others. They get a full customs inspection yearly, instead of every three years like other participating companies.

In one 24-hour period in April, customs officers in Laredo found three tons of marijuana in trucks carrying auto parts across two different bridges. Five days later, agents in El Paso, Texas, found more than four tons of marijuana in a tractor-trailer hauling auto parts.

In July, the director of the program became alarmed by the number of large drug seizures along the border and issued a security bulletin asking participating companies to redouble their efforts against smuggling.

Stephen Flynn, senior fellow for Counterterrorism and National Security Studies at the Council on Foreign Relations, said truckers did not feel safe rejecting bribes, no matter what agreements their companies have made with the U.S. government.

"The basic vulnerability for a truck driver remains the 'plata-or-plomo' dilemma," Flynn said, using Spanish shorthand for taking a bribe or a bullet.

John Chaffin, a trade lawyer near San Diego, suspects that participating companies feel pressure from drug gangs to help them smuggle drugs into the United States.

"Some Mexican truckers have figured out, 'I don't want someone thinking I'm a better target than someone else,' " Chaffin said.

Roberto Ramirez de la Parra, then chief of operations for Mexico's customs agency, told El Norte newspaper that exporters last year became worried that organized crime was targeting U.S.-certified companies.

Daniel B. Hastings Jr., owner of a customs house with offices at five ports of entry on the Texas-Mexico border, thinks the customs program works. He cited cases in which a Mexican trucking company tracking a truck noticed an unscheduled stop en route to the bridge and phoned to alert U.S. customs.

Said Hastings: "I think they're doing as good as they can with what they have to work with."