BOSTON — A madman armed with a large butcher knife terrorized commuters at the Forest Hills MBTA station Friday morning, threatening to stab people and slashing at bus tires before fighting with transit cops in a struggle so violent that an officer's gun went off.

MBTA Transit Police Superintendent Richard Sullivan said officers received a report around 11:15 a.m. of an armed man threatening people with a butcher knife at the busy Jamaica Plain station. He was also attempting to stab the tires on a bus.

Officers already on scene responded immediately. What happened next was chaos.

The suspect fought with officers and grabbed for one of their weapons, Sullivan said. During the struggle, the officer's gun discharged. Nobody was struck by the round, but an investigation will attempt to determine exactly how the firearm went off.

"This was a violent person armed with a butcher knife that was threatening someone to stab them and was trying to stab tires on a bus," Sullivan said.

Two transit police officers were injured in the brawl — one hurt his knee, the other suffered minor injuries. Both were taken to the hospital and are expected to be OK.

The suspect, an adult male whose name has not been released, is now in custody. He is expected to be arraigned next week and will face at minimum assault and battery with a dangerous weapon charges, according to Sullivan.

Boston EMS said six people total were transported from the scene by ambulance. Sullivan was unable to immediately explain why four additional people beyond the two officers were hospitalized.

Aerial footage showed over a dozen police vehicles and two ambulances swarming the station, with yellow police tape cordoning off a large section of the upper busway. The MBTA temporarily shut down the Forest Hills Upper Busway, directing riders to board all buses from the lower level. Orange Line service was not affected.

At this point, the only thing more predictable than MBTA delays is the next violent attack on the system.