The cause of the fire wasn't immediately known. Workers first saw smoke coming from a storage room about 5 p.m.

Chicago Fire Commissioner James Joyce told the Associated Press that the building had an alarm system but no sprinklers above the first floor. The employees who died appeared to be from one stairwell around the 22nd floor, ten stories above the source of the fire, according to Joyce.

As many as 2,500 people work in the high-rise, but many had already left for home by the time the fire started.