Fire, attempted suicide ground flights at Chicago's O'Hare airport
United Airlines passengers checking in at Kennedy International Airport Jan. 6, 2014 in New York [AFP]

A fire at an air traffic control center outside Chicago on Friday led to the cancellation of all flights at the city's O'Hare International Airport, one of the world's busiest, and domestic hub Midway, snarling air traffic nationwide ahead of weekend travel.

More than 800 flights into and out of the two airports were canceled by 9:20 a.m. CDT (1420 GMT), according to tracking website, stranding thousands of passengers.

O'Hare is the main hub for United Airlines and a major hub for American Airlines. From January to August, more than 580,000 flights departed or landed at O’Hare, the city of Chicago said, citing Federal Aviation Administration data.

Southwest Airlines Co suspended all flights until noon at Midway and at Milwaukee’s General Mitchell International Airport, the airline said in a statement. It was not clear what other airlines planned to do.

The Chicago Department of Aviation said employees at the FAA control center in Aurora, Illinois, were evacuated when the fire broke out before 6 a.m. CDT (1100 GMT).

"The Chicago enroute center has been evacuated due to a fire in the facility. This has resulted in a ground stop for flights in the area and heading to Chicago," the FAA said in a statement.

Airspace management has been transferred to adjacent air traffic facilities, it said.

Crews responding to the fire found a man with self-inflicted wounds in the basement of the facility, the Chicago Tribune reported. NBC News, citing two sources at the FAA, said officials believe the fire may have been intentionally set.

The man was taken to a hospital, the Tribune said, citing an Aurora Police Department spokesman.

Representatives for the police department could not be immediately reached for comment.

On May 13, about 700 flights were canceled at O’Hare and Midway airports after a faulty motor in the heating and cooling system at a flight control center in Elgin, Illinois, a Chicago suburb, caused smoke to circulate and forced staff to clear out of the building.

Flight operations at the Elgin center were transferred to the Aurora control center and limited arrivals and departures resumed more than three hours after the incident.

(Reporting by Colleen Jenkins, Karen Brooks, Mary Wisniewski and David Baliey; Writing by Jon Herskovitz; Editing by Scott Malone, Susan Heavey and Mohammad Zargham)