It happened around 2:30 Thursday morning at the Beechmont Condominium Complex between Main Street and Beechmont Avenue in Bridgeport.
The heavy smoke and flames sent residents running for their lives. Some jumped from terraces on the first and second floor.
Thirty-one residents made it out on their own, but firefighters rescued one elderly occupant, who was unconscious in his apartment.
Officials say they had to act quickly because of the design of the building, Truss Construction with a common roof, is especially dangerous in a fire.
The heat of the flames also damaged the offices of a law firm next door.
The two story apartment building will likely be demolished. The 13-apartment structure was built in 1988.
Fire marshals are investigating the cause.
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