CLAYTON • A St. Louis County police officer with more than 20 years on the force used excessive force when handling a prisoner and could face criminal charges, Chief Tim Fitch said Thursday.

Supervisors believed the officer acted inappropriately when dealing with an intoxicated prisoner at the West County Precinct on July 18, so they launched an internal investigation, Fitch said.

"We didn't just shove this under the rug," Fitch said. "We're policing ourselves. We didn't hear from this prisoner. We reached out to him. I think we're sending a message to other officers."

The officer in question decided to retire during the internal investigation, Fitch said.

County police turned in its findings to the prosecutor's office last week for consideration of criminal charges. Surveillance video of the incident was included, said Officer Rick Eckhard, the department spokesman.

The officer's name will not be released unless charges are filed, officials said.

Fitch said the alleged victim was intoxicated and threatening officers. "Unfortunately that does not justify the officer's actions, which were outside of what we deem as an acceptable use of force," Fitch said.

He would not elaborate about the alleged assault but said the prisoner did not suffer any injuries during the incident.