Mexico’s cartels have for years diversified into a wide variety of illicit activity, helped by porous domestic law enforcement agencies as well as long-standing trafficking routes into the United States, their biggest market.

Meanwhile, opioid deaths in the United States have soared over the last two decades, driving a wave of government-backed efforts to disrupt illegal distribution and treat addicts.

The DEA said that 27% of a sample of counterfeit pills tested in the United States during the first three months of this year contained potentially lethal doses of fentanyl.

The DEA did not specify what pills were being faked, but photos of what it said it had seized showed mostly blue tablets stamped with the letter “M” on one side, and containing the number “30” on the other, similar to a brand of opioid oxycodone hydrochloride.

“Capitalising on the opioid epidemic and prescription drug abuse in the United States, drug trafficking organisations are now sending counterfeit pills made with fentanyl in bulk to the United States for distribution,” the DEA’s acting head Uttam Dhillon said in a statement.

Dhillon added that pills containing fentanyl and fentanyl-laced heroin cause thousands of deaths every year in the United States.