Washington — U.S. prosecutors have charged two Chinese nationals who sold fentanyl to American customers over the internet in a massive international conspiracy case, the Justice Department announced on Tuesday.

The case is unique, the Justice Department said, because the men are the first Chinese-based fentanyl manufacturers and distributors to be slapped with a label reserved for those who have control of the most prolific international drug trafficking and money laundering organizations.

Prosecutors identified the Chinese men charged in the case as 40-year-old Xiaobing Yan and 38-year-old Jian Zhang.

Yan, the Justice Department alleged, operated websites selling fentanyl directly to U.S. customers, and he also ran at least two chemical plants in China that were capable of producing tons of fentanyl and fentanyl analogues. Zhang, the department alleged, similarly ran an enterprise that made fentanyl in at least four labs in China, and he advertised and sold fentanyl over the internet.

At a news conference to announce the charges, Deputy Attorney General Rod Rosenstein said the cases “mark a major milestone in our battle to stop deadly fentanyl from entering the United States.” The men were labeled Consolidated Priority Organization Targets and are the first Chinese-based manufacturers to be given the designation.

It is unclear whether the men could ever be brought to the U.S. to face charges. Rosenstein said China has no extradition treaty with the U.S., though U.S. authorities would share the evidence in the case with their Chinese counterparts and were “optimistic” that the Chinese would take some sort of action.

Rosenstein said he was in China two weeks ago meeting with China’s minister of public security about fentanyl and other issues.