A few miles from the St. Louis suburb where Michael Brown died in August, another black teenager was fatally shot by a police officer on Tuesday night, setting off a flurry of tense confrontations between protesters and the police on Wednesday.

The authorities in the region moved quickly to distinguish the shooting from Mr. Brown’s case and others, which have set off protests over police conduct and race around the nation. Officials from the St. Louis County Police Department, which is investigating the shooting, said the teenager, Antonio Martin, pointed a gun at the police officer, who is white, before being shot outside a gas station late Tuesday night in Berkeley, Mo. The officials said a handgun was found at the scene, and they released surveillance video of portions of the encounter.

Theodore Hoskins, the mayor of Berkeley, a mostly black city of about 9,000, said that the city would also conduct its own investigation, but that from his initial review of the video recording, it appeared that the officer was threatened and did not open fire until a gun was pointed at him.