A 20-year-old man has been charged with assaulting a St. Louis light-rail passenger who says he was attacked after being asked about the "Mike Brown situation."

St. Louis city prosecutors charged Ronald Williams Tuesday with misdemeanor assault, a day after he and a 15-year-old were arrested in connection with the March 23 attack of a 43-year-old commuter.

The case went viral after a passenger who wasn't involved posted video of the attack on social media.

The victim told authorities a man asked to use his cellphone. When the victim refused, he said he was asked what he thought about the Brown "situation," referencing the fatal shooting of the black unarmed 18-year-old by a white police officer in the suburb of Ferguson.

It wasn't immediately clear if Williams has an attorney.