The president of the Officers Union said the officer acted appropriately, despite what's seen in the video of the officer putting the man's head through a glass window, which shattered it.
"You want to stay away from the edges of the platform. The train can move at any time. If the train moves and you end up in an altercation and you end up on the track-way, all bets are off," said Jesse Sekhon with the Officers Union.
The officer involved in the incident is on medical leave. He suffered a concussion and multiple cuts that required stitches.
BART says it's too soon to tell if their officer used excessive force. They are still interviewing witnesses and going over 911 calls made from other passengers that evening.
The suspect, 37-year-old Michael Gibson of San Leandro, suffered cuts to his hand and forearm, which may have happened when he collided with the glass.
Gibson was arrested earlier that day, for misdemeanor assault on an 82-year-old man on a bus in Daly City.
His family says he has bipolar disorder and schizophrenia. The family's attorney said a lawsuit will be filed.