The smoke at the Elgin Terminal Radar Approach Control, or TRACON, building forced its evacuation Tuesday. As a result, flights into and out of O'Hare and Midway airports were grounded.
Federal Aviation Administration spokesman Tony Molinaro said controllers returned after the smoke was cleared about three hours later. They immediately went to work clearing delayed arrivals or departures.
As flight operations resumed, departures were delayed about 90 minutes Tuesday at O'Hare and 2 hours at Midway.
Officials said about 1,000 fights were canceled at O'Hare and 120 at Midway. AP