A Ferguson, Missouri, police officer fired the fatal shot that killed a man at a gas station in the St. Louis suburb, police said Wednesday.

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The shooting happened just before 1:30 a.m. Tuesday at a convenience store. Police said 26-year-old Detarius Haynes and 18-year-old Joshua Campbell were in a vehicle that pulled up to a gas pump. St. Louis County police said in a news release that Campbell shot Haynes, who collapsed and died near the gas pump. It wasn't immediately clear what prompted that shooting.

A Ferguson officer who was walking out of the store shot Campbell. He ran onto West Florissant Road, where he collapsed and died, St. Louis County police said.

The 37-year-old officer was unhurt.