Authorities did not know how the fire started in the 35-story building, which houses county and state offices. The fire broke out on the 12th floor.
The trapped people were found in stairwells and hallways from the 16th to the 22nd floors, above where the fire broke out near the end of business hours. Fire officials evacuated them and were continuing to search the building.
"We continue to search, we are going square foot by square foot. — People tend to hide from the fire, we are looking," Fire Commissioner James Joyce told a reporter for CBS affiliate WBBM.
"Some were conscious and breathing. Some were not," Joyce said. He said some of those trapped may have been overcome by smoke, but had no additional information on their conditions.
The Cook County Medical Examiner's office confirmed three deaths in the fire.
Joyce said he did not know how the fire started in the building, which has an alarm system but no sprinklers.
A number of people initially escaped on their own. Firefighters escorted others down stairways and evacuated a daycare center without incident, fire officials said.
But about an hour after the fire appeared to be under control, more than a dozen ambulances were called back to the scene and rescuers could be scene removing victims on stretchers.