Investigators are waiting for things to cool down before they start looking for what started the blaze in the four-story multi-family building on Bergenline Avenue Monday afternoon.
The building contains several apartments and has businesses on the ground floor.
Fire officials believe the blaze began on the fourth floor and was fueled by a tar roof.
The Red Cross is working to relocate about 100 residents who lost their homes.
Luckily, no injuries were reported.
The battle took place more than 40 feet in the air, and for most of Monday afternoon and evening, it looked as if the fire was winning.
Newscopter 7 captured water cannons blasting flames that were ripping through the building.
It took hours before firefighters got it under control.
"The fire got to a very advanced stage early in the day, so we're trying to figure out what happened here," North Hudson Fire Chief Frank Montagne said.
The Union City Mayor's Office is collecting donations for the displaced fire victims. To help, please contact:
3715 Palisade Ave
Union City, NJ
The Union City Building Department said there were 23 apartments in the building, as well as a supermarket and a hair salon on the ground floor.
The building was built in 1928 and is owned by 5th Street Bergenline LLC, with the same address. The building is 18,000 square feet.
Three other neighboring buildings were evacuated as a precaution.
Get Eyewitness News Delivered