More than 100 firefighters responded to the building near West Maypole Avenue and North Western Avenue.
Firefighters had to strategize how to put the fire out because of all the smoke. They also said a group of people inside may have started the fire.
"Somebody said they saw kids running away as the generator came out. It was hard to get to and see and ventilate," one firefighter said.
One firefighter was treated for heat exhaustion, but is expected to be OK.
Investigators are still trying to figure out how the fire started.