Story highlights An African-American man in his twenties beats a white man after asking about Michael Brown

Two more young black men join in the beating, which is caught on cameras

(CNN) When one man sat down next to a second man in a St. Louis light rail car and asked him his opinion on the shooting of Michael Brown, it was not the beginning of a discussion.

It was the start of an assault, police said.

The second man, who was white, didn't want to answer the question. Then the first man, who was black, boxed him in the face. Two more African-American men joined in the beating, according to a police report about Monday's incident.

It was caught on surveillance cameras on the MetroLink train and a passenger recorded it with a cell phone and posted the video online. It has gone viral.

Police confirmed to CNN that the online images came from the attack.

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