The fire at a Chicago-area FAA air traffic control center was started by a contractor, police told the Chicago Tribune.
So far about 1,600 flights have been grounded across the United States, disrupting routes on Southwest Airlines, American Airlines and United Airlines. Southwest has said that it won't resume service at Chicago Midway or Milwaukee until at least 7 p.m.
70 percent of Midway's flights and 39 perfect of flights at Chicago O'Hare have been cancelled.
The disruption could wind up costing airlines as much as $50,000 per canceled flight, according to USA Today.
"The cost to airlines will be tens of millions," Mike Body, president of aviation consulting firm Boyd Group International, told USA Today. "Probably in the high tens of millions depending on how long the grounding goes."
Losses will mostly stem from operational expenses associated with readjusting crew schedules.
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