The blaze broke out around 12:30 a.m. at the Villa, an apartment building on Prospect Street in East Orange, and was still burning more than eight hours later.
Heavy flames quickly spread through the four-story structure and through the roof, spewing smoke that could be seen for miles.
Sections collapsed as firefighters battled the fire, and dozens of residents were evacuated before the fire was finally declared under control at 9 a.m.
The Essex County Prosecutor's Office confirmed that an elderly woman died in the blaze.
Her name has not yet been released.
Mullti-alarm fire ravages East Orange apartment building
Eyewitnesses described a chaotic scene.
"I saw people running up the street to Prospect Street, and they're like, 'There's a fire, there's a fire in the back of the building,'" one man said. "And I went out on my fire escape, and that's when I saw the building go up in flames."
Five firefighters were treated for minor injuries at University Hospital, while six residents were treated by EMS at the scene.
Mayor Ted Green's office said the fire displaced 18 families.
The East Orange Office of Emergency Management and the American Red Cross are providing direct aid to the victims.
A temporary shelter has been set up a block from the fire scene at Edward T. Bowser, Sr., Elementary School, located at 180 Lincoln Street.
The cause of the fire is under investigation.
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