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Denis Poroy, File, Associated Press
FILE - In this June 21, 2005, file photo, U.S. Customs and Border Protection Inspector Maria Arreola checks an electronic ID card at one of two new Secure Electronic Network for Travelers Rapid Inspection lanes at the San Ysidro Port of Entry in San Diego. The two new lanes allow pre-screened participants to use an automated identification system to cross into the U.S. from Mexico more rapidly. SENTRI users are being targeted by drug smugglers who place magnetized boxes under the cars of unsuspecting drivers.

SAN DIEGO — 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.