14-alarm fire levels an entire township block in Belleville, N.J. Here's how it may have started.

Sunday's 14-alarm fire in Belleville, New Jersey, took out an entire block. Several dozen fire departments were needed to put out the blaze.

Two firefighters were treated overnight for smoke inhalation and exhaustion, but, miraculously, no one else was hurt.

Twenty hours after the fire broke out, smoke was still visible on Monday afternoon and the air quality wasn't good. Mayor Michael Melham said the fire started in a mattress factory at 347 Cortlandt St., and spread to each side of the building.

What's left looks like complete devastation. A building with many businesses and two homes are gone.

Many of the people who were evacuated were allowed back in, but the people who lost their homes are being helped by the American Red Cross.  

How the fire may have started

Melham said construction at the mattress factory likely had something to do with sparking the blaze.

"We believe it was some welding that was being done and when you weld near fabric or foam, that's a bad scenario right there," Melham said.

He said it was the perfect storm in the warehouse -- helium tanks and a cardboard company were there, too. Flames were seen shooting out of windows and embers flew two blocks away, destroying two homes.

"Because it was windy, that ember was flying throughout the air and landing on rooftops," Melham said.

An upholstery business was also destroyed.

"To see everything gone, all of our work gone, it's depressing, honestly," warehouse employee Angel Lauriano said.

"I heard the explosions"

Residents across the street barely escaped the Sunday afternoon inferno. Elena Gonzalez said she ran out of her home with her five dogs.

"I heard the explosions. I heard the walls fall down," nearby resident Alan Panella said.

"The first thing we took out [was] them, because it's the most important thing we have," an emotional Gonzalez said. "They are our babies, and I don't care about another thing."

"The firefighters told everyone [to] get out faster because it's going to be coming here," resident Jeff Oliver added.

"It doesn't feel real"

Inside the building, the Legacy Boxing Gym, which owner Yessenia Montalvo says empowered girls and trained Olympic contenders, is now nothing but ashes.

"It is a family, and I mean you can tell [from] the messages on our Facebook page, on our Instagram, everybody just giving us support and, like, it's not about me. It's about what we've built, how we support one another," Montalvo said.

She said the gym was full hours before the fire.

"We heard commotion next door," Montalvo said.

"It doesn't feel real, if am being honest," boxing trainer Richard Guzman said, adding "We have to" keep fighting.