UPDATE | Witnesses said teen had his hands raised when he was shot in Mo.

FERGUSON, Mo. (AP) — A black teenager who was fatally shot by a police officer had his hands raised when the officer approached him with his weapon drawn and fired repeatedly, according to two men who said they witnessed the shooting, which sparked a night of unrest in suburban St. Louis.

The FBI opened an investigation today into the death of 18-year-old Michael Brown, who police said was shot multiple times Saturday after being confronted by a white officer in Ferguson, a 21,000-resident suburb that's nearly 70 percent black.

Authorities were vague about exactly what led the officer to open fire, except to say that the shooting was preceded by a scuffle of some kind. It was unclear whether Brown or a man he was with was involved in the altercation.

Investigators have refused to publicly disclose the race of the officer, who is now on administrative leave. But Phillip Walker said he was on the porch of an apartment complex overlooking the scene when he heard a shot and saw a white officer with Brown on the street.

Brown "was giving up in the sense of raising his arms and being subdued," Walker told The Associated Press today. The officer "had his gun raised and started shooting the individual in the chest multiple times." The officer then "stood over him and shot him" after the victim fell wounded.

Dorian Johnson offered a similar account when he told WALB-TV that he and Brown were walking home from a convenience store when a police officer told them to get out of the street and onto the sidewalk. Johnson said they kept walking, which caused the officer to confront them from his car and again after getting out of his car.