Police who got there quickly knocked on doors and got the more than 80 residents out of nearly three dozen apartments in the four-story building at the corner of 4th and Bergen streets, Fire Chief Patrick Trentacost said.
"We pulled up and got the lines in place fast," Trentacost said, noting that the blaze began in an upper floor stairwell and spread to the roof.
"This type of fire gets away very quickly into what is known as the cockloft," the fire chief said. "We protected the cockloft, it did not get in there, and the fire, in this case, went out pretty quick."
As firefighters doused the two-alarm blaze, the Church of God across the street "opened its doors and let the families come in," Mayor Hector Carlos Lora said.
Other tenants were temporarily put up at No. 8 school, he added.
The mayor called it a "cold but blessed morning."
"Because people identified the fire and called immediately, we were able to respond right away and take care of everything," the mayor said.
Hours later, city officials were relocating some families while getting others back into the building.
Arson investigators were summoned to determine the cause of the fire.
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