UPDATE: BFD says a short circuit started the fire.
Boston firefighters responded around 10:40 p.m. to 72 Beacon St., for a fire in the ceiling of the top floor of the four-floor building.
The fire went to two alarms before it was declared knocked down shortly after 11 p.m.
One firefighter suffered a back injury, the Boston Fire Department reports, adding a second firefighter was taken to a local hospital for observation.
Five residents were displaced, the department reports.
The cause is under investigation.