In some cases, the smugglers will stuff the cocaine in metal ammunition containers, weld them shut, and then stow the boxes in the gas tank of a tractor-trailer truck. That makes it difficult for the dogs to sniff out.
Other traffickers will build a false ceiling in the back of their big rigs, creating a six-inch cavity that makes a perfect lair for hundreds of pounds of marijuana.
Most, though, are more straightforward: They simply hide the narcotics with the other cargo, often trying only to conceal the smell with onions or some other vegetable.
Along one of the most porous borders in the world for narcotics, the difference between a bust and another successful shipment of cocaine can come down to the cunning of traffickers, the nose of a dog, or a bit of serendipity for those in law enforcement.
Each day tens of thousands of trucks now pass into the US from Mexico - 5,000 through Laredo's checkpoints alone. The traffic, which has increased exponentially since the passage of the North American Free Trade Agreement, has given smugglers a virtually unlimited number of ways to spirit narcotics into the US.
A decade ago, critics predicted that NAFTA would lead to increased drug trafficking along the 1,100-mile border. Today, there's at least some evidence that they're right: Narcotics are pouring in.
The US Customs Service, for instance, has tripled the amount of drugs it has seized since NAFTA became a reality in 1994. Even more dramatic, the US Border Patrol confiscated 352 percent more marijuana last year from trucks than it did in 1999.
"Our seizures continue to go up, but that doesn't seem to be stopping or putting the hurt on [drug traffickers]," says John Smietana Jr., who oversees the Border Patrol's anti-smuggling unit in Laredo. "It just means a whole lot more dope is getting though. If we catch 10 percent of it, they see that as the price of doing business."
Nowhere is the problem more pronounced than in this bustling border town, where almost half of all goods entering the US come through. Though other border areas complain of finding increasing amounts of drugs, Laredo is the busiest inland port in North America - and the main portal for trafficking by truck.
Some of the increase in seizures, of course, is simply due to having more officers along the border. Both the Border Patrol and the Customs Service have doubled their budgets and staffing in the past 10 years. And as more drugs are confiscated, law-enforcement authorities have hard numbers to use in lobbying for resources.
At the same time, the increase in narcotics trafficking in part reflects the increased flow of everything across the border - people, cars, commerce. Indeed, supporters of NAFTA like to point out that passage of a free-trade law alone can't lead to more drugs. It takes someone to use them.
"The argument that NAFTA has caused an increase in drug trafficking by making it easier to smuggle across is ignoring the fact that it's an industry driven by demand," says J. Michael Patrick, director of the Texas Center for Border Economic Enterprise Development at Texas A&M International University here.
Yet those on the front lines of trying to stop the drug trade say the queue of trucks that line up each morning at major checkpoints has complicated their jobs - and certainly made it easier for smugglers to do theirs.
The problem falls particularly hard on state and local law-enforcement agencies, which often don't have the resources to deal with the deluge of drugs.
"We have one pair of night-vision glasses in our office," says Martin Cuellar Jr., who oversees the Texas Department of Public Safety's (DPS) narcotics unit in Laredo. "But the crooks on the opposite side of the river have one pair each."
Mr. Cuellar shifts in his chair behind his desk in Laredo. Down the hall, his officers are interviewing a man arrested in a 1,100-pound marijuana bust at a nearby warehouse in March. The drugs were trucked across and then brought to a warehouse on the US side, where another truck was scheduled to pick them up and take them north.
Cuellar says NAFTA can be correlated to the increase in drugs coming in because he's heard it from informants on the Mexican side. In fact, a well-known drug cartel moved from Ciudad Juarez to Nuevo Laredo last year, and informants say its mainstay is truck-trailers.
One and a half million trucks a year through Laredo's port of entry is "like a gold mine to them," says Cuellar. "[Smugglers] know that you can't stop something that's good for the economy."
Others see ominous signs as well. Michael Scott, chief of the DPS's criminal division, says some organizations have purchased trucking companies and maquiladoras in Mexico to provide legitimate cover for their operations.
"It is clear that there have been serious unintended or overlooked consequences" from NAFTA, he says.
Some, however, see the claims of increased trafficking by truck as overblown. Tom Wade, president of the Laredo Transportation Association, says many of the arguments are simply an attempt to keep Mexican trucks from being able to travel freely in the US. At the moment, they're confined to border areas.
Still, as more big rigs do come across, even within these restricted areas, law-enforcement agencies are being forced to be more vigilant. Customs officials say of the thousands of trucks that cross daily, they might flag 200 to be X-rayed. Of those, maybe 20 are unloaded and sorted through. In total, fewer than 10 percent of vehicles are searched.
"This is a prescription for failure," says Mr. Scott. "Mexican drug-trafficking organizations have exploited our collective inability to inspect vehicles and pedestrians entering this country."
Though they can't check every trailer, Customs officials try to make up for it with trained eyes. On this day at the World Trade Bridge in Laredo, four Customs inspectors work the line of trucks that snakes out of sight. They're looking for abnormalities.
They start with the cargo manifest - the paper that says who the driver is, what the cargo is, and where it's going. They might find discrepancies there.
They ask the driver questions, checking for signs of nervousness. They scan the exterior of the truck. A shiny screw might indicate that something has been tampered with. Sometimes they open the back of the trailer. The type of cargo can tip them off. So can an empty trailer.
While this is going on, "Persie" and his handler are cruising up and down the line of trucks. The drug-sniffing dog stops at the cab, the gas tank, the doors to the trailer. Police say dogs are the key to the whole operation. The Border Patrol in Laredo estimates that they're responsible for 95 percent of its seizures.
Back on the Customs dock, Mexican workers reload a trailer. Planters, leather chairs, Mexican pottery, buckets, and metal stands are strewn about. A search like this might take 3 or 4 hours.
No drugs are found, but the cargo was probably flagged because the X-ray machine couldn't detect what was inside the metal buckets, says Francisco Garcia, the World Trade Bridge's chief Customs inspector. When asked if his crew is able to catch all the drugs coming across, he adopts the cautious language of someone involved in a daily war. "I can say yes, and I can say no," he says simply.
(c) Copyright 2001. The Christian Science Monitor