An Arkansas officer was fired after video footage was released showing a man being punched and elbowed while handcuffed in the back of a police car.
The Jonesboro Police Department said in a Facebook post on Friday - the day after the incident occurred - that Joseph Harris' termination was "effective immediately".
"The serious nature of the complaint necessitated prompt action," the post said. The department also posted a video of the incident.
Jonesboro Police Chief Rick Elliott told the Associated Press he was "shocked and appalled", and said he would refer the case to prosecutors.
The incident was brought to Mr Elliott's attention by a complaint filed by the county sheriff's office.
The department then conducted an internal review and released the video footage of the encounter "in the interest of transparency".
In the 12-minute video, a man detained in the back of a patrol car is seen wearing a hospital gown and telling police, "I have fentanyl inside me".
He says he made the same complaint to nurses at the hospital where he was before, but "they wouldn't listen to me".
The man then appears to attempt to strangle himself with a seatbelt strap before the car stops. Police said Mr Harris is seen opening the back door of the vehicle, and repeatedly punching and elbowing the man in the face.
The video then shows the door slamming, apparently hitting the man's head.
Chief Elliott subsequently fired Mr Harris and said he would be referring the case to the local prosecutor. Mr Harris has not been criminally charged.
Mr Elliott also contacted the FBI’s Little Rock office and will ask the state to decertify Mr Harris as a police officer, according to the Associated Press.
“Wrong is wrong. There’s not really anything to investigate,” Mr Elliott said.
The BBC has contacted the Joneseboro Police Department for comment.