BILLINGS (AP) — A federal judge sentenced the leader of an interstate drug ring to 20 years in prison Friday for his role in a major trafficking operation that stretched from western Washington to the Bakken oil fields of the Northern Plains.

U.S. District Judge Susan Watters said Robert Farrell Armstrong, of Moses Lake, Washington, preyed upon addicts in the Montana-North Dakota oil patch to build up a network of drug couriers, dealers and armed enforcers.

Armstrong, also known as “Dr. Bob,” was arrested in October as part of a law enforcement crackdown aimed at the Bakken oil region’s proliferating market for illicit drugs. Authorities have struggled to curb rising crime rates in once-quiet rural communities as people flocked there to take part in the region’s new prosperity.

Armstrong, 50, pleaded guilty in January to a single count of possession of methamphetamine with intent to distribute. He’s the 13th defendant sentenced in the case. Five more defendants have pleaded guilty and are awaiting sentencing, and another is scheduled to go to trial in November.

Authorities say the Sidney-based drug ring moved as much as a pound of meth a week before it was busted up last year.

“You’re an old criminal, Mr. Armstrong, and you’ve been doing this a long time,” Watters said. “The community needs to be protected from you.”

Before hearing his sentence, Armstrong asked Watters for mercy and apologized to the court for the damage he had caused to his family and people in Montana.

He said his judgment had been clouded by living for many years as a drug addict and he has been sober now for almost a year. “I believe prison is going to be good for me,” Armstrong said at one point.

Similar drug rings have been broken up in neighboring North Dakota. Along with meth and heroin, the criminals moving into the oil patch have brought weapons and a willingness to engage in violence, authorities say.

Armstrong came to the region in 2012 from Washington, where he had a string of drug and assault convictions stretching over three decades, according to Assistant U.S. Attorney Joe Thaggard.

In Montana’s oil patch, Armstrong set up shop in a trailer along the Yellowstone River and oversaw a “massive, well organized scheme” that brought in large volumes of almost-pure meth from Washington state for distribution, Thaggard said.

Other Montana cities and towns where the ring distributed meth were Fairview, Billings, Big Timber, Columbus, Livingston and Bozeman, authorities said.

Armstrong’s network included couriers to get the drugs from Washington, dealers to sell it in Montana, and enforcers — some with firearms — to collect on overdue drug debts, said Jeff Nedens, an agent with the Montana Division of Criminal Investigation.

Nedens said Armstrong also traded meth for guns with at least one customer.

Under Armstrong’s plea agreement, prosecutors agreed to dismiss additional drug conspiracy, distribution, and firearms charges.

Armstrong’s public defender, Anthony Gallagher, asked Waters for a 10-year sentence. Gallagher argued that prosecutors overstated Armstrong’s leadership role in the trafficking ring and his criminal history.

He pointed out that Armstrong and at least one other co-defendant had been supplied with meth from the same source — a supplier in western Washington identified in court only as “George.”

“This was the classic case of a group involved collectively in a criminal enterprise,” the defense attorney wrote in a pre-sentencing memorandum. “All or almost all were independent contractors working in concert to obtain methamphetamine.”

Watters said she was not convinced.

Armstrong “was in fact the person who recruited these people in eastern Montana to distribute these drugs, to go on these drug runs,” the judge said. “The defendant was the connection between George and all of these people.”

Nedens said George and other unspecified individuals remain under investigation.

Prosecutors have racked up 105 indictments since the Bakken drug crackdown began about two years ago, U.S. Attorney Michael Cotter said this week. Another 100 indictments, all on federal narcotics charges, are expected in the next 12 months, Cotter said.