Two Chinese men have been charged with manufacturing and selling “tons of fentanyl” and other powerful synthetic drugs, according to the Justice Department. 

It’s the latest development in the United States’ desperate efforts to plug the supply of these potent drugs that are driving up the overdose rate. 

On Tuesday, DOJ officials announced the federal indictment of 40-year-old Xiaobing Yan and 38-year-old Jian Zhang—accusing the men of running two separate enterprises that they say led to four deaths.

Zhang is charged with running “at least four labs” that produced the drugs and sold them via the web.

Yan’s operation was discovered after a 2013 traffic stop in Mississippi that allowed authorities to trace him up the supply chain. Yan is said to have operated “at least two chemical plants” in China that had the capacity to manufacture “tons of fentanyl.”

Authorities allege that Yan was diligent in keeping his operation legitimate. They say he would monitor drug legislation and law enforcement efforts in the United States, and adapt the chemical composition of his drugs to skirt anti-drug laws.

Despite U.S. authorities’ supposed revelations, according to the AP, they won’t likely be able to bring Zhang and Yan to the U.S. to face the charges, in the absence of an extradition treaty with China.

Robert Patterson, who stepped in as acting DEA administrator after Chuck Rosenberg stepped down last month, said these international drugmakers present “one of the most significant drug threats facing the country.”

Over the last year, the U.S. government has singled China out as the main source for fentanyl and synthetic opioids many times stronger than heroin, like carfentanil, which is traditionally used as a tranquilizer for large animals like elephants.

Last year the DEA announced that China has become the “primary source” of fentanyl in the U.S., contributing to rising overdose rates. 

Former Health and Human Services Secretary Tom Price, who recently resigned amid accusations that he abused his private jet privileges on the taxpayer’s dime, said over the summer that he was confident in China’s cooperation with the U.S. on anti-drug efforts, saying the nation has been “an incredible partner in helping to stop the production of drugs like fentanyl.” 

However, Yu Haibin of China’s narcotics control agency isn’t as optimistic about the “fight” against drugs: “My feeling is that it’s just like a race and I will never catch up with the criminals.”