Drug smugglers have discovered a new tactic for getting their wares across the border – via unsuspecting couriers responding to help wanted ads in Mexican newspapers.

Under the ruse, prospective employees who can legally cross the border (a job prerequisite) apply for a job in janitorial or restaurant work in Tijuana and are told to take a company vehicle and report to a job site in San Diego. Upon arriving, they’re told there will be no work for them that day, receive a token payment of between $50 and $200, and are told to leave the vehicle and walk back across the border.

It’s not disclosed to the job seekers that their vehicles are loaded with drugs, which makes them both appear less nervous when dealing with border security and considerably cheaper than professional smugglers, who routinely take in $1,500 or more per trip.

The Associated Press reports that 39 such arrests have occurred at San Diego’s two border crossings since February 2011, with seizures totaling 3,400 pounds of marijuana, 75 pounds of cocaine, and 100 pounds of methamphetamine.

Now, U.S. Immigration and Customs Enforcement is fighting the phony advertisements with ads of their own warning job seekers about the scam. Mexican authorities, however, doubt such warnings will act as much of a deterrent to those desperate for work, who will be unlikely to do much to vet an employer before accepting work.

The cases, which the U.S. attorney’s office will not discuss in detail, can be difficult to prosecute, as the smugglers themselves are for the most part unaware that they’re transporting drugs, and thus cannot be charged with intent to commit a crime.