BALTIMORE (WJZ) — Five Baltimore firefighters were hurt in the line of duty. City fire crews battled a four-alarm fire Monday at a lumber company when the building collapsed and trapped them inside.

Kai Jackson has the latest on the massive blaze.

Sky Eye Chopper 13 was above the raging four-alarm fire in the 600-block of Pennsylvania Avenue, a potentially deadly inferno that appeared to start on the first floor. WJZ captured dramatic video as firefighters loaded an injured comrade into an ambulance.

“This was a pretty big fire,” said Kevin Cartwright, Baltimore Fire Department.

The business, which was closed at the time, is listed as Penn Lumber. Firefighters say it was fully involved when they arrived, as seen in mobile phone video captured by an eyewitness.

“We heard a loud poof and the whole front of the building collapsed,” said witness Cassius Evans.

Firefighters say at some point, the building became unsafe and they signaled for all the firefighters to retreat. As they did, two floors of the building collapsed on top of them and a Mayday was called to rescue them.

“There were five firefighters that did suffer some injuries who were actually trapped beneath the lumber, the bricks from this building that actually came down from the second and third floor on top of them,” Cartwright said.

Over the next several hours, firefighters will do what’s known as a “fire watch.” It means staying vigilant for any hot spots. They’ll knock those down and allow the investigation to continue.

The wounded firefighters were taken to Shock Trauma with non-life threatening injuries.

About 100 firefighters battled Monday night’s blaze.