The three-alarm fire started at 11:15 a.m. Wednesday at 311 William Street, which is a three-story home.
PSE&G says a leak in an underground gas main fed the flames.
The gas spread to surrounding buildings, and an entire city block was evacuated.
Firefighters finally extinguished the fire, though the battle was made more difficult because of a broken underground gas main in front of the home feeding the fire.
Two residents of the home were hospitalized. Two firefighters were also injured. One firefighter fractured his ankle after jumping out a first floor window.
12 people were evacuated from 311 William Street. Two other 30-unit apartment buildings and nine two-family homes were also evacuated.
The fire was contained to the one house.
Most of the residents are either staying with friends or family or being put up at the nearby Ramada Inn by the Red Cross.
There is no word on when they will be allowed back home.
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