WATERTOWN, N.Y. -- A fire at a vacant building on Newell Street in Watertown sent two firefighters to the hospital.

Officials say they responded to the scene shortly after 6 p.m. Monday.

Crews worked for hours to get the flames under control when part of the building collapsed, trapping the two firefighters before other crew members pulled them from the wreckage.

One of the firefighters is being treated at Upstate University Hospital in Syracuse for a possible hip fracture and internal injuries, but is expected to recover, while the other was evaluated at Samaritan Medical Center and sent home.

Tuesday morning, crews continue to monitor the structure and are putting out a number of hot spots as they flare up.

"It's just a lot of hidden spots. You can't get to it, you can't get inside in order to put it out, you really gotta be standing on top of it.  Without doing that, we're just waiting for the hot spots to flare up and do the best we can to put water on it and wait for the investigation to be over. The only time we're going to really get this out is when it finally comes down,” said Watertown Fire Department Battalion Chief Tucker Wiley.

The roads surrounding the building were closed for much of the night, but have since reopened. The building is owned by Newell Street Realty, LLC.

The fire is still under investigation and the cause is unknown.  State fire investigators say they're planning to use a drone to survey the damage and further their investigation.

The heavy smoke and flames drew plenty of onlookers to the scene Monday night.

Spectrum News spoke with two kids riding their bikes nearby, who said the smoke made it hard to see and breathe. They watched what they described as a chaotic scene, as the injured firefighters were pulled from the collapsing building.

A couple from Utica who used to live in the area were also there, saying they've never seen a fire like this.