Prosecutors in Tulsa, Okla., on Monday filed homicide charges against a 73-year-old Sheriff’s Department volunteer who fatally shot a suspect on April 2, apparently firing his handgun instead of a Taser by accident as other officers were subduing the man on the ground.

The volunteer, Robert C. Bates, was charged with second-degree manslaughter involving culpable negligence, punishable by up to four years in prison.

Mr. Bates, an insurance broker, had been a reserve deputy since 2008. He is among scores of civilian police enthusiasts, including wealthy donors to law enforcement, some of whom effectively act as an armed adjunct to the Tulsa County Sheriff’s Department.

Sheriff’s officials said he had intended to subdue the fleeing suspect, Eric C. Harris, with a Taser, but mistakenly fired his handgun instead. Mr. Harris was accused of trying to sell an illegal gun to an undercover officer.