The victim -- reportedly in his 80s -- was trapped after the fast-moving blaze quickly engulfed his East 23rd Street home shortly before 3:30 p.m., responders said.
A neighbor tried to get to him but was pushed back by the flames and smoke, they said. The roof in the rear of the building collapsed soon after, turning a rescue into a recovery.
The Passaic County Prosecutor's Office, which is investigating the cause, hadn't officially released the victim's name as of Monday morning.
"We mourn this tragic loss," the Paterson Fire Department wrote in a social media post Sunday night.
The Red Cross helped three other residents with food, shelter and clothing.
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VIDEO: Paterson Fire 5/16/2021 (Fireground Buff Network)
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Authorities were trying to determine the cause of the fire.
The Passaic County Prosecutor's arson unit was summoned to the scene.
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