(CNN) -- All flights into and out of Chicago's O'Hare International Airport and Chicago Midway International Airport were stopped Tuesday.
The ground stop was in effect because of smoke in a radar facility, according to a statement from the Federal Aviation Administration.
"All FAA personnel were evacuated from the Chicago Terminal Radar Approach Control (TRACON) facility in Elgin because of smoke in the building. All TRACON operations were immediately transferred to the Chicago En Route Center in Aurora," it read.
TRACON air traffic controllers take control of long distance flights as they approach. They then guide traffic to just a few miles from the airport, before handing off to a controller who handles landings on a runway.
The cause of the smoke was not immediately clear.
CNN's Mike Ahlers and Thom Patterson contributed to this report.
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