WASHINGTON, United States (AFP) – An internal police investigation was underway in St. Louis, Missouri Thursday after a white officer shot and killed an African-American teen not far from where a similar shooting in August led to weeks of unrest.

St Louis Police Chief Sam Dotson said the off-duty officer was moonlighting as a security guard when he shot and killed the 18-year-old, who they said opened fire first.

Police said the young man killed in the incident was one of a trio of suspects who caught the officer’s attention as he drove around a St Louis neighborhood late Wednesday in a private security patrol vehicle.

At one point, the officer got out of the car, the youths took off running, and the officer gave chase.

“One of the gentlemen started to approach the officer in an aggressive manner,” the police chief told reporters at a news conference early Thursday. “The officer was giving them verbal commands, telling them to stop,” he said.

Dotson said that the suspect and the police officer, a six-year veteran of the force, got into a scuffle. The teen then took flight again, at which point the officer noticed for the first time that the youth was carrying a handgun.

As he ran away, “the suspect pointed the gun at the officer and fired at least three rounds at the police officer,” Dotson said.

“At that point the officer returned fire… as he was being shot at,” the police chief said, adding: “The suspect is deceased.”

He said police said they recovered a nine-millimeter handgun at the scene, but news reports said relatives of the youth dispute the police account, and insist that the teen was unarmed.

Local media said the officer fired a total of 17 shots, but it was not clear how many of them struck the teen.