Two men in China have been charged by American federal prosecutors as being the kingpins of a vast international conspiracy to manufacture and sell fentanyl, a powerful opioid, via unnamed Dark Web sites.

In a press conference in Washington, DC, on Tuesday, Deputy Attorney General Rod Rosenstein said that the bust marked the first time the Department of Justice had "indicted major Chinese fentanyl traffickers."

"Chinese fentanyl distributors are using the Internet to sell fentanyl directly to US customers," Rosenstein continued.

He also said:

They use multiple identities to disguise their activities and their shipments and to obscure the trail of profits going back to China. They take advantage of the fact that the fentanyl molecule can be altered in numerous ways to create a fentanyl analogue that is not listed as illegal under US and Chinese law. When regulators are able to identify the new fentanyl and make it illegal, the distributors quickly switch to a new, unlisted fentanyl analogue.

The Centers for Disease Control and Prevention said recently that fentanyl can be 50 to 100 times more potent than morphine, and it is often mixed with heroin or cocaine. The drug, which is legally prescribed for pain relief, can be highly addictive.

The two men, Xiaobing Yan, 40, and Jian Zhang, 38, are believed to still be in China—the DOJ did not say whether it had begun extradition efforts. However, the United States and China have no formal extradition treaty, so getting the men across the Pacific to face their charges may be difficult.

Investigators believe that Yan, who was indicted in federal court in Mississippi, has been producing methylone and other related substances and selling them in the United States. The case was brought there because a routine traffic stop in the Magnolia State resulted in the discovery of a "domestic drug ring that sold various synthetic cannabinoids, sometimes dubbed "spice" or "bath salts.""

According to Rosenstein, federal authorities "identified more than 100 distributors of synthetic opioids involved with Yan’s manufacturing and distribution networks."

Zhang, by contrast, was prosecuted in North Dakota, following the 2014 death of 18-year-old Bailey Henke.

"Agents were able to identify another Grand Forks resident as the source of the fentanyl," Rosenstein continued. "This individual had been using the crypto-currency Bitcoin to buy fentanyl, heroin, and other drugs over the Internet for more than a year, using an encrypted website on the Dark Web. Dogged agents and prosecutors traced the source of the illegal drugs through Oregon, Canada, and eventually to Jian Zhang in China."

In addition to the Chinese leaders, there are numerous other co-conspirators, including five Canadians and three American residents. A total of 21 people have also been charged in Oregon and North Dakota as part of this alleged conspiracy.

Ars has reported on the bust of online methylone and fentanyl dealers before—but DOJ did not immediately respond to Ars' questions as to whether they were related.

The Tuesday announcement comes roughly three months after AlphaBay, one of the world's largest online drug websites, was seized and shuttered. Numerous prosecutions stemming from AlphaBay's closure are underway in California and elsewhere.