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Traffic in the US enters Mexico at the San Ysidro border crossing, the world's busiest, on June 27, 2008 in Tijuana, Mexico.
More than five million trucks crossed into the U.S. from Mexico last year. For drug smugglers, getting a truckload of illegal narcotics past border authorities means potentially huge profits.
But they're often up against two levels of security: that of U.S. law enforcement, and that of private export and shipping companies. Reporter Jill Replogle from the Fronteras Desk explores how authorities and trucking companies are trying to stay ahead of smugglers.