A U.S. program that offers trusted trucking companies speedy passage across American borders has begun attracting just the sort of customers who place a premium on avoiding inspections: Mexican drug smugglers. Most trucks enrolled in the program pause at the border for just 20 seconds before entering the United States. And nine out of 10 of them do so without anyone looking at their cargo. The government keeps the list of participants secret, citing national security and trade secrets. More than half of all U.S. imports now come from companies in the program, called the Customs-Trade Partnership Against Terrorism, or...