U.S. prosecutors charged 10 people, including four Chinese nationals, with selling large quantities of fentanyl to Americans as part of an alleged international money laundering conspiracy, Attorney General Jeff Sessions announced Friday.

It was a five-year conspiracy that involved 30 separate aliases, such as “Phantom Pharma” and “Toxic Storm,” according to an indictment. The suspects used encrypted communications, cryptocurrency, laundered funds and offshore accounts, Sessions said.

Separately, the Treasury Department froze U.S. assets belonging to the conspiracy’s alleged leader, Jian Zhang, 38, also known as “Hong Kong Zaron,” who was previously indicted last year, along with the assets of four other Chinese nationals charged with crimes by the Justice Department. It marked the first time the United States has taken such steps against alleged fentanyl traffickers.

Treasury also sanctioned Zhang’s Chinese biotechnology company, which means that any of the company’s assets in the United States or in the possession of an American must be blocked and reported to the federal authorities.

The Chinese-based network moved more than 400 grams of fentanyl, a powerful synthetic opioid, and more than 100 grams of a fentanyl analogue in the U.S. and Canada, according to two federal indictments filed in North Dakota.

“Fentanyl and its analogues killed more Americans than any other drug in 2016, and the vast majority of it comes from China,” Sessions said in a speech to law enforcement officials at the Quentin N. Burdick U.S. Courthouse in Fargo, N.D. “Synthetic opioids like fentanyl killed 20,000 Americans in 2016.”

Sessions said the defendants allegedly trafficked fentanyl from China to 11 states, a pipeline stretching from Oregon to Ohio to Florida. The investigation began in Grand Forks, N.D., with the fentanyl overdose death of 18-year-old Bailey Henke in January 2015, Justice Department officials said.

“This was an elaborate and sophisticated conspiracy,” Sessions said. “It killed people in North Carolina, New Jersey, Oregon and Grand Forks.

“This office opened an investigation and traced the source of the fentanyl all the way back to China,” Sessions said of acting U.S. Attorney Christopher C. Myers’s office. The defendants allegedly shipped fentanyl and its analogues through the mail.

Prosecutors identified the Chinese nationals charged in the case as Na Chu, 37; Yeyou Chu, 36; Cuiying Liu, 62 and Keping Zhang, 62. The United States considers them fugitives, and it is unclear whether they could ever be extradited to face charges.

The indictments Friday are part of a case announced by Deputy Attorney General Rod J. Rosenstein last October when he revealed the names of the first Chinese-based fentanyl manufacturers and distributors to be charged as “consolidated priority organization targets,” a label used for those who control the largest international drug-trafficking and money-laundering organizations.

So far, 32 people have been charged in the case. Steven Barros Pinto, 36, was arrested in Illinois, Keveen Odair Nobre, 28, was arrested in Rhode Island and Robinson Andres Gomez, 25, was arrested in North Carolina.

The two leaders of the network in Canada operated from a medium-security prison where they were incarcerated in Quebec, according to the indictment.

In his speech, Sessions urged Americans to dispose of unwanted prescription drugs on Saturday by participating in the Drug Enforcement Administration’s National Prescription Drug Take Back Day. The DEA program allows people to get rid of prescription drugs with no questions asked, he said. The last two DEA Take Back days pulled in 900 tons of drugs, he said.

“I have no doubt that saved lives and kept these drugs from getting into the wrong hands,” Sessions said. “We are not just talking about numbers. We are talking about moms, dads, daughters, spouses, friends and neighbors.”