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. 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.