UPDATE 4:20 p.m.: The motorcyclist in today's Mass Ave. bridge accident has died. State police confirmed the 20-year-old man was pronounced dead at the scene.
He was struck and killed by an SUV on the bridge. State police said the driver of that truck was examined for emotional distress. The cause of the accident remains under investigation.
UPDATE 3:30 p.m.: Boston.com reports that the motorcycle operator was taken to Brigham and Women's Hospital. There's no word on his or her condition. Steve Bognar from WBZ news, meanwhile, tweeted this photo of the motorcycle on the bridge, describing the scene as "very bloody."
ORIGINAL POST: Boston police and first responders are treating an accident victim on the Mass Ave. bridge. The motorcyclist was struck by a car and is in traumatic arrest, according to police radio transmissions.
Crews are treating the victim and a haz-mat unit has arrived to the scene. The bridge is currently closed indefinitely.
This is a developing story. Back Bay Patch will update this post as more information becomes available.