This gives us a pretty good picture of who is moving across the border, said Frank Longoria, a Customs officer who is assistant director of field operations for border security. Ninety-nine percent of people who cross are doing so for good reason, but trying to catch that 1 percent that is doing something illegal is challenging.

In a small building not far from the entry and exit lanes, a Customs officer, Eugene Jimenez, looked at an X-ray scanning system, which allows him to see anomalies in the frame of a vehicle. He said he was looking for spaces where there should be solid material, or obvious signs of tampering in the gas tanks, batteries or other areas.

A few days before, after a currency detection dog reacted to a white 2008 Volkswagen Passat traveling into Mexico, Mr. Jimenez noticed a space in the bumper when the car was pulled aside for a scan, he said.

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On the Mexican Border, a Case for Technology Over Concrete - New York Times