Two Chinese nationals are facing federal prison and millions of dollars in fines after allegedly spearheading a global drug trafficking scheme selling fentanyl over the dark web.

Indictments unsealed this week (17 October) accused 40-year-old Xiaobing Yan and 38-year-old Jian Zhang of manufacturing and selling the opiate to US customers online.

Police intercepted packages mailed from Yan's internet-based pharmaceutical companies which contained "multiple kilograms" of suspected fentanyl – which the Department of Justice (DoJ) noted was "potentially enough for "thousands of lethal doses."

The two men, who allegedly operated at least four chemical labs in China, allegedly built up a network of more than 100 North America-based distributors.

US law enforcement agents said that, in total, 21 people now face charges as part of the distribution probe.

The indictments were filed in the Southern District of Mississippi and the District of North Dakota.

An investigation into the internet-based drug scheme, maintained by Yan and Zhang, was launched after a number of fentanyl deaths in the two states.

"Chinese fentanyl distributors are using the internet to sell fentanyl directly to US customers," said deputy attorney general Rod Rosenstein. "They use multiple identities to disguise their activities and their shipments and to obscure the trail of profits going back to China."

"We are working closely with our colleagues in China and other countries to stem the flow of illegal fentanyl into the United States," he added.

Rosenstein said that an "encrypted website on the dark web" which had been used to purchase fentanyl was later traced directly to Zhang in China.

Ultimately, federal agents said that Zhang had sent "many thousands" of illicit packages since January 2013. His organisation marketed the substances online and sent fentanyl and other drugs through the mail or international parcel delivery services, the DoJ said.

Both men remain based in China and are not currently in US custody. There is currently no extradition agreement between the two countries.

The US government said that more than 20,000 Americans were killed by fentanyl and fentanyl analogues in 2016. "Fentanyl is 50 times more potent than heroin and 100 times more potent than morphine," asserted ICE acting deputy director Peter Edge.

Yan was indicted on two counts of conspiracy to manufacture and distribute multiple controlled substances and seven counts of manufacturing and distributing drugs. If convicted, he would face up to 20 years in US prison and a $1m fine.

Zhang was indicted for conspiracy to distribute fentanyl and fentanyl analogues in the US and international money laundering charges. If nabbed by US law enforcement, he will face up to life in prison and $12.m in fines.

The US DoJ said that five Canadian citizens, two residents of Florida and a resident of New Jersey were also indicted in the District of North Dakota this month.

"We live in an increasingly global and interconnected world – crime has no borders," said Joanne Crampton, assistant commissioner of the Royal Canadian Mounted Police (RCMP).

"Law enforcement must respond accordingly by working beyond our borders together to detect and disrupt criminal activity. By fostering an integrated and coordinated law enforcement approach, we will continue to disrupt [...] drug trafficking networks."

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