LOUISVILLE, Ky. — An investigation into an alleged prostitution ring in Kentucky was traced to a veteran officer in a small-town police department, where the patrol supervisor was arrested on rape, sodomy, assault and other charges as he reported for his night shift, authorities said Wednesday.

Oak Grove Police Sgt. Benjamin R. Walden, an overnight supervisor in the southwestern Kentucky town bordering Fort Campbell Army post, seemed stunned when Kentucky State Police troopers took him into custody late Tuesday at the city’s police department, his chief said.

“I told him to come in (early), that I needed some extra help for a warrant,” Oak Grove police Chief Dennis Cunningham said in a phone interview. “I didn’t tell him that it was his warrant. He showed up and he was taken into custody when he walked in the door.”

Two alleged accomplices were arrested five days before Walden, state police said.

The investigation began when state police were tipped off by an alleged victim, Trooper Sean Wint of the state police said. Troopers found that three adult women were being held against their will and forced to have sex with men at an Oak Grove motel, he said.

Walden joined the Oak Grove police force as a patrolman in 2012 and was promoted to sergeant in 2015. Walden previously worked for the nearby Hopkinsville police department from 1996 until 2011.

Walden, 43, of Clarksville, Tennessee, faces multiple charges, including first-degree sodomy, first-degree rape, fourth-degree assault, permitting prostitution, retaliating against a participant in a legal process and tampering with a witness, state police said.

Other charges against him include official misconduct, terroristic threatening and intimidating a participant in a legal process.

“During the investigation and speaking with the victims, we were able to get a lot of information on some incidents that had occurred that made those charges appropriate for him,” Wint said.

Wint didn’t rule out the possibility of more arrests as part of the continuing investigation.

Also arrested in the case were Michael Helton, 34, of Oak Grove, and Kiersten Napodano, 22, of Joelton, Tennessee, state police said. They are charged with unlawful imprisonment, human trafficking and promoting prostitution, Wint said. Helton also is charged with possession of a handgun by a convicted felon.

All three have pleaded not guilty and are being held in jail. Their attorneys didn’t immediately return calls seeking comment Wednesday.

Cunningham, the Oak Grove police chief, said state police contacted him Tuesday about Walden’s alleged involvement. He said his department is cooperating fully with investigators.

The chief described Walden as a “good officer” and said the arrest didn’t reflect badly on his 15-officer department in the town of about 8,000.

“Regardless of what your job is, you’re still expected to act within the law,” Cunningham said, adding that his officer is innocent until proven guilty. “Just because you carry a badge for a living doesn’t put you above that.”

Walden’s arrest comes nearly two weeks after a former Oak Grove police officer and another man were acquitted in the slayings of two brothel workers 22 years ago. The two men were acquitted of murder in the cold-case deaths of two women who were shot and stabbed New Life Massage Parlor in Oak Grove in 1994.