FARGO, N.D. — A man accused in an international fentanyl-trafficking operation that led to the overdose death of a North Dakota man pleaded not guilty Tuesday to allegations he ordered other defendants to keep quiet.

Daniel Vivas Ceron, a Colombian national who allegedly ran the enterprise from a Canadian prison, is charged with obstruction of justice. An indictment says he wrote letters telling co-conspirators to remain silent and alerting other suspects about the investigation.

Vivas Ceron, 37, was charged earlier this year with five counts, including aiding in the distribution of fentanyl that resulted in the January 2015 death of 18-year-old Bailey Henke, of Grand Forks. Two Chinese nationals are also charged in what authorities are calling one of the world’s most prolific fentanyl-trafficking and money-laundering operations.

U.S. Attorney Chris Myers and defense attorney Brian Toay declined to comment after Tuesday’s hearing. Toay said he’ll know more details about the direction of the case after he and prosecutors meet with U.S. District Judge Daniel Hovland on Thursday. Trial is scheduled for Feb. 6.

Vivas Ceron asked U.S. Magistrate Judge Alice Senechal if he could get involved in the obstruction investigation, by questioning witnesses and challenging the government’s evidence. The judge told him he was better off talking with his lawyer before saying something that could be incriminating.

“You’ve been appointed an attorney to speak for you in court and file motions with the court,” Senechal said.

Vivas Ceron also told Senechal that he struggles understanding English at times.

“I’m trying to do my best,” he said.

The federal case in North Dakota has been dubbed Operation Denial. Bailey’s death was traced to fentanyl that Vivas Ceron allegedly shipped to Brandon Hubbard, a Portland, Oregon, man who told police he could be the largest fentanyl dealer in the country. Hubbard was sentenced last year to life in prison.

Further investigation led to the Justice Department announcement last month in Washington D.C. — attended by Myers — that two men from China were charged with separately running chemical labs that produced fentanyl and other illegal opioids that were sold to Americans. More than 20 people have been indicted in the case.

Fentanyl can be lethal even in small amounts and is often mixed in with heroin and other drugs. The Centers for Disease Control and Prevention estimated that more than 20,000 Americans were killed by fentanyl and its analogues in 2016.