A 20-year-old man was charged Tuesday with assaulting a St. Louis commuter who reported he was beaten after being asked about the officer-involved shooting death of Michael Brown.

Ronald Williams was charged with misdemeanor third-degree assault. However, St. Louis prosecutors decline to pursue the case as a hate crime against the 43-year-old victim. Cellphone video of the attack went viral after it was posted on social media.

Williams, who is black, was arrested Monday with a 15-year-old boy. The boy’s status was unclear Tuesday because he is a juvenile under state law.

The victim, who is white and described the criminal complaint only as “D.A.,” told police that he declined a man's request to use his cellphone while riding the Metrolink train. The stranger then  asked the victim what he thought about “the Michael Brown situation.”

The victim said he had not given much thought about the Ferguson matter, which prompted the 20-year-old to repeatedly punch the victim in the face, police said.

The video also shows two other men joining in on the attack before they got off the train.

Rachel Smith, a city prosecutor, told The Associated Press on Tuesday said she passed for now on pressing the case as a hate crime because it lacked a key, provable threshold for it under Missouri law — that the alleged attack involved racial or religious motivations.

The misdemeanor count carries a possible year in jail and $1,000 in fines; prosecuting the matter as a hate crime would enhance that punishment.

"At the present time, we are still evaluating motive," she said. "I'll just say that at this time, this is what we have."

Smith credited tipsters with helping identify the suspects in an attack she suggested authorities can't overlook.

"Someone simply minding their own business and riding public transportation, they have an expectation that they should travel without harassment or bodily harm," she said. "We all need to view this situation seriously and as an affront to all of us."

Williams' bond was set at $2,500 cash.

The Associated Press contributed to this report