Chicago police are now investigating to determine if foul play was involved.
Officials thought the fire had been struck out on Wednesday, but on Thursday morning, it started up again.
On Saturday, as demolition crews worked to tear down the structure for a second day, the flare-ups continued.
The Chicago Fire Department has said the official cause for the fire was "open flame ignition of available combustibles."
The building is owned by developer Calvin Boender, who was found guilty of bribing Chicago Ald. Ike Carothers to get a zoning charge.
