BILLINGS (AP) — Drug convictions in cases tied to the Bakken oil patch continue to pile up with four more men recently sentenced to federal prison terms for trafficking drugs in eastern Montana.

The latest sentencings announced Friday included 2-1/2 years in prison for Keith Edward Lester of Sidney, and more than three years in prison for Sean Michael Vaira of Billings.

Both men had pleaded guilty in late 2013 before U.S. District Judge Susan Watters to charges involving methamphetamine distribution.

The defendants were linked by authorities to a drug ring that brought in large quantities of meth from the West Coast and sold it in towns across eastern Montana.

More than 20,000 people have poured into eastern Montana and western North Dakota since oil production in the region began its meteoric rise in 2008.

Authorities say drug dealers have been moving in to profit off the boom. Drug arrests in the Bakken region more than tripled over the past several years, according to the Montana Board of Crime Control.

Federal, state and local officials launched a crackdown last year, but officials say they’ve struggled to keep pace with the rise in drug activity.

An offer of proof from the U.S. Attorney’s Office said that beginning in December 2012, the 30-year-old Vaira was involved in the sale of more than 2  1/2 pounds of meth in Sidney and Big Timber.

Lester, 44, told authorities that he started distributing methamphetamine in the Sidney area after his son introduced him to the alleged ring-leader, Robert Armstrong, in September 2012, according to the U.S. Attorney’s Office.

At least 10 other defendants have pleaded guilty in related cases.

That includes Armstrong, who is due to be sentenced by Watters in May.

Two other recent cases were linked by authorities to the oil patch.

On March 5, 48-year-old Anthony Rungo of Roseburg, Ore. was sentenced to more than 10 years in federal prison after pleading guilty in December to possession with intent to distribute meth in a case before U.S. District Judge Donald Molloy.

And on March 6, Roger Allen Berger of Dickinson, N.D., was sentenced to almost six years in prison by Watters.

He pleaded guilty in November to conspiracy to possess meth and heroin with the intent to distribute.