MEXICO CITY >> Former President Donald Trump and other Republicans have blamed President Joe Biden’s border policies for the fentanyl pouring into the United States, playing on a widespread belief that immigrants entering the country illegally are responsible for bringing it in.

In reality, the largest known group of fentanyl smugglers is not made up of immigrants traversing the desert or moving through secret tunnels — they are Americans coming through legal ports of entry. More than 80% of the people sentenced for fentanyl trafficking at the southern border are U.S. citizens, federal data shows.

Since 2019, when Mexico overtook China to become the dominant supplier of fentanyl in the United States, cartels have been flooding the country with the synthetic opioid. The amount of fentanyl crossing the border has increased tenfold in the past five years. Mexico has been the source of almost all of the fentanyl seized by U.S. law enforcement in recent years.

Officials say those numbers point to a new and alarming strategy: Mexican drug cartels are turning thousands of Americans into fentanyl mules, deploying a torrent of couriers who can easily cross back and forth into their own country.

“Americans are taking Mexicans’ smuggling jobs,” said David Bier, a border security expert at the Cato Institute, a libertarian think tank. “The cartels are really using U.S. citizenship as an asset that they can leverage.”

Bars, gyms, rehab facilities, trailer parks — these are all places where recruiters have found couriers in recent years, court records show. A college football star was lured in by a friend after dropping out of school. A mother raising three special-needs children took the job while facing eviction. A homeless man was recruited from an encampment in a Walmart parking lot.

Federal agents uncovered a recruitment network inside more than a dozen San Diego high schools.