UPDATE: WBZ reports two of the firefighters have died.
298 Beacon St. Nine alarms. Firefighters from Engine 33 were trapped without water. Three taken out with injuries, one in critical condition.
Prosecutors from the Suffolk County District Attorney's office are now on scene, standard procedure when a victim has life-threatening injuries.
At one point, commanders ordered all firefighters out of the four-story brownstone, built in 1871 and once owned by Oliver Wendell Holmes.
The order came not long after a dispatcher lost contact with the Engine 33 firefighters, who issued a Mayday call.
Boston EMS reports a total of 18 people have been transported to local hospitals.
Department chaplains were summoned to both the fire scene and Mass. General, where at least one firefighter was brought.
Beacon Street and Storrow Drive are shut. BFD estimates it could be several hours before the fire is completely put out.
Engine 33, based in the historic Boylston Street fire house, was one of the first BFD units to respond to the Marathon bombings last year.
The fire from Kendall Square (photo by Chia Evers):
Photo by BFD: