Eighteen people were taken to the hospital, including firefighters, though officials have yet to release specific details about the nature of their injuries.
The current Boston EMS patient transport count for the Beacon Street Fire is now at 18.— Boston EMS (@BOSTON_EMS) March 26, 2014
"All of sudden, the smoke got black and pushed out in the street and I heard a big explosion noise," an eyewitness told WCVB.
Authorities did say that they had accounted for all the residents in the building.
The Boston Fire Department released several photos of the incident on its Twitter page.
They identified the dead as Lt. Edward J. Walsh, 43, and Firefighter Michael R. Kennedy, 33. Walsh had three children, while Kennedy was a veteran of the U.S. Marine Corps.
Notable deaths of 2014 [PHOTOS]
Cyclist struck and dragged underneath car