Smoke in a regional radar facility forced a halt to all incoming and outgoing flights at both of Chicago's airports.
The Federal Aviation Administration says all its personnel were evacuated from the Chicago Terminal Radar Approach Control, or TRACON, facility in suburban Elgin, Illinois, at around 11:30 a.m. Tuesday.
Spokesman Tony Molinaro says a ground stop is in effect for all flights in and out of O'Hare International and Midway airports.
He noted that inbound flights already in the air over the area were handled by a backup facility in the city of Aurora, Illinois.
FAA maintenance personnel and the Elgin Fire Department determined that a faulty motor in the HVAC system cause the smoke, and there was no fire, according to an FAA statement. Officials are working to clear up the remaining smoke in the radar room so that air traffic controllers can return to work.