Mexican drug traffickers have begun to set up laboratories to produce the deadly synthetic opioid fentanyl, US Attorney General Jeff Sessions said Thursday.
China up until now has been the main source of fentanyl -- up to 50 times more potent than heroin and much cheaper to produce -- shipping large amounts through Mexican narcotics networks to the US market.
"Fentanyl originally started mostly from China," Sessions said at the close of a US-Mexico bilateral meeting on transnational criminal organizations.
"A considerable amount has been shipped to Mexico and then enters across the border in some fashion from Mexico."
Now, though, he added, "We are also seeing precursor chemicals in Mexico and manufacturing labs begin to develop in Mexico. So one of the priorities I would like to see us do is to nip that in the bud, stay very intensely focused on those laboratories, and make sure that it does not become a big problem in the future."
Fentanyl is increasingly the main cause of death in the US opioid crisis, which saw an estimated 64,000 people die last year.
The epidemic has its roots in the massive over-marketing of highly addictive prescription opioids like Oxycontin as painkillers.
Moves to tighten that market have forced addicts to turn to heroin and, increasingly, cheaper but more lethal fentanyl.
"We've never seen those numbers before, or anything like it," Sessions said of the overdose death rate.
The United States has also put pressure on China to crack down on laboratories there exporting fentanyl to the US market, via multiple channels including the regular mail and parcel services.