Authorities warn that smugglers are using powerful magnets to turn tourists into drug mules in Mexico.

The drug smugglers are hiding drugs on the bottom of drivers’ cars. The drivers targeted have been part of the “trusted traveler” program, which allows drivers who have passed background checks to skip the line and quickly cross the border.

Smugglers then retrieve the drugs once back inside the U.S.

Read more from the Associated Press:

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.