Drug smugglers are turning "trusted travelers" into unwitting mules by placing containers with powerful magnets under their cars in Mexico and then recovering the illegal cargo far from the view of border authorities in the United States.

One motorist spotted the containers while pumping gas after crossing into Southern California this month, and thought it might be a bomb. His call to police prompted an emergency response, and then a shocker: 13 pounds of heroin were packed inside.

There have been four such incidents in San Diego since Jan. 12, all involving drivers enrolled in the federal "trusted traveler" program, which enables hundreds of thousands of people who pass extensive background checks to whiz past inspectors with less scrutiny.