The blaze broke out in the complex, being built at the intersection of St. Georges Avenue and Stone Street, just after 1:30 a.m.
The spectacular flames lighted the sky and roused neighbors.
"I heard the helicopters flying over. I said what's going on out here. And as I looked out, I seen the flames shooting from the top of the building," Kevin Retcho said.
"I seen fire coming out from the roof and stuff - through the windows. It started out on the left side corner of the building, then it just started working itself around the whole building," Omar Cadiz said.
In fact, the blaze was so intense it took firefighters from ten surrounding departments to help put it out.
Because of the tight modular construction of the unoccupied building, dousing hot spots was an all day job.
Arson investigators joined firefighters on the scene trying to determine how and why the fire started.
By late afternoon, they did know where it started.
"If you're looking at the building, it's what we would call the B side, which is the left side and about halfway back." Rahway Fire Chief Bill Young said.
This complex had been under construction for about a year. It was almost completed and set to open as rentals this spring.
The fire chief says it's not salvageable.
"The whole third floor is gone. There's mortar damage. The whole building is gone," he said.
No injuries were reported. Calls of the smoke smell came from as far away as Manhattan.
