WOONSOCKET, R.I. (WPRI) — An investigation is underway into the cause of a fire that destroyed a Woonsocket apartment building Wednesday afternoon.

Smoke could be seen billowing from the roof of the Elm Street apartment building for hours as firefighters worked to knock down the flames.

Woonsocket Fire Captain Robert Cahill said 20 people, including 12 adults and eight children, have been displaced.

It’s believed the fire started on the second floor, as that’s where the bulk of the damage was and where firefighters were met with heavy flames.

“When they first got inside there, there was heavy fire on the ceiling area,” Cahill said. “[It’s a] very extensive house … very compartmentalized. [We] had to work to get to the fire.”

Firefighters called in backup from a number of nearby communities, including Central Falls, Lincoln, North Providence and Smithfield.

Woonsocket police said one resident is being treated for smoke inhalation after he ran back inside the burning building to save his pets on the first floor.

Police said a dog and cat that lived on the third floor died in the fire.

“Everybody should think about losing everything in just a blink of an eye,” said Richard Rainville, one of the property managers. “They lost it in a flash.”

“Six families, [that’s] a lot of kids,” he continued. “It just saddens my heart that these families are out of a house.”

The cause of the fire is under investigation and the displaced residents are being helped by the American Red Cross.

Corey Welch contributed to this report.