Firefighters are still working to put out the fire at the abandoned Alpha Beta meat packing plant at 303 S. Santa Fe Ave. in Pueblo.
They continued to work all day on Monday.
The blaze started Sunday night between 10 p.m and 11 p.m. Firefighters believe it started in the first floor, then burned up an exterior hallway, through a doorway, and into the third floor.
Firefighters weren't able to get into the building when the fire first started. They said the building was in poor condition and difficult to navigate, especially through the heavy smoke. They finally went in Monday morning.
The cause of the fire is still under investigation, but investigators suspect a transient was inside the building and may have started a fire to keep warm. Firefighters said if there were any people inside, it's not likely they survived.
The Pueblo Fire Department asks that citizens do not call 911 regarding smoke in the area.
One firefighter was injured Sunday night when he tripped and fell. By Monday, he was back home recovering.