CHEYENNE (AP) — A sixth man pleaded guilty on Thursday to felony drug charges in the heroin overdose deaths last year of a professional bull rider and two others in Cheyenne.

Christopher Charles Tyson, 34, of Cheyenne pleaded guilty on Thursday to distributing the heroin and cocaine that killed 21-year-old bull rider Bryan Guthrie last December.

Tyson also pleaded guilty to conspiring with others to move the drugs that killed Guthrie and two others who died earlier last year: 34-year-old Valarie Anne Sena and 32-year-old Martin Christopher Gerhold. The Associated Press previously has reported Sena’s age incorrectly.

Tyson faces 20 years in prison when sentenced by U.S. District Judge Alan B. Johnson in November. The judge will review a presentence report before formally accepting Tyson’s plea.

In exchange for Tyson’s guilty pleas, federal prosecutors dismissed a charge that he used a firearm in the commission of drug crimes. Tyson had been scheduled to go to trial next week. If convicted, he could have faced life in prison.

Tyson is the sixth man to plead guilty to drug charges in the case, including two who entered pleas Tuesday. Federal officials are awaiting extradition of a seventh man from Canada.

Cheyenne lawyer Dion J. Custis represents Tyson. Custis said after the court hearing that the case was complex and said he was fairly happy with the result for his client.

John Powell, spokesman for the U.S. Attorney’s Office in Cheyenne, said Thursday after Tyson’s plea hearing that all the related cases have been difficult. “We’re just happy to see it come to some kind of conclusion,” Powell said.

Last year’s overdose deaths and the ensuing investigation uncovered a drug distribution network in which Tyson and other Cheyenne residents commonly drove to Denver to buy heroin from dealers. Once back in Cheyenne, Tyson and other defendants have said they would use some of the drug themselves and sell the rest to subsidize their habits.

Guthrie was ranked as high as third in bull-riding in January 2009 in the Professional Rodeo Cowboys Association. He was knocked out of competition with a leg injury in March 2009.

Guthrie’s death in particular shocked Wyoming, where heroin use — until now — has been foreign to its traditional cowboy culture.

U.S. District Judge William F. Downes of Casper in June sentenced defendant Joel D. Murdoch of Cheyenne to 21 years in prison for his role in the drug distribution conspiracy. Murdoch is the only defendant to be sentenced in the case so far.

The Casper Star-Tribune reported Downes said at Murdoch’s sentencing that he used to spend time around cowboys and, at the time, thought their only vices were chewing tobacco and whiskey.

“I always thought cowboys were different,” Downes said. “When I learned cowboys were dying of heroin, I was incredulous.”