The incident, which led demonstrators to take to the streets to protest police violence, came two months after a wave of riots shook the St. Louis suburb of Ferguson following the shooting death of an unarmed black youth, Michael Brown, by a white policeman.
According to the police version of Wednesday's shooting, the dead youth was one of three people who fled after being approached by the officer, who was working at a department-approved secondary job for a private security company.
Police chief Sam Dotson told a press conference that the suspect was armed and fired three shots while being chased by the officer, and they had recovered a gun at the scene.
The off duty policeman, who was uninjured, fired back 17 times at the suspect, killing him.
Police have not identified the officer or the dead young man.