A variety of businesses were located in the building, located in the 2600 block of west Nelson, that's near Belmont and Elston in the Avondale neighborhood.
A hot spot was discovered 18 hours after the huge warehouse went up in flames. The hoses were working nonstop as the walls of this massive structure were picked apart.
The fire started just after 1:30 Sunday morning and because of the building's size, at first, firefighters only saw smoke.
"When we got here they had a little trouble trying to find inside building. It was a maze, five buildings in one," said CFD Chief Michael Fox.
Quickly flames appeared and it was clear a lot more help was needed. In all, it took over 200 firefighters and more than 80 pieces of equipment to contain the blaze.
The fire spread through the top three floors of the building causing a wall to collapse. The 400-foot by 400-foot building is an old factory that was turned into a multi-use warehouse.
"They have different companies working there like a wood shop, metal shop, they lampshades in the back, they have different shops there," said Edwin Vega, who works in the building.
Craig Sellers had a custom design cabinet business on the first floor. Not only did the fire destroy his company, but his delivery van was crushed.
"I'm a little in shock right now trying to figure out how I'm going to start over," Sellers said.
Fortunately, the fire did not spread to a two-story building that is attached to the warehouse.
While those business owners were spared, Ned Machic says there has always been concern about a fire because he says the fire sprinklers did not work in the entire structure.
"We had asked the owners of the building, they didn't do anything about it, so I know the sprinkler system wasn't working, for sure," Machic said.
The fire department did not confirm the information about the sprinklers.