Passengers make their way through a terminal at O'Hare International Airport in Chicago, Illinois, September 26, 2014. [Photo/Agencies]

DELAYS THROUGH THE DAY

The incident caused major delays at O'Hare and the domestic hub Midway International Airport, affecting flights from almost every state and routes with Europe, Asia and Latin America.

There were 1,755 flights into and out of the two airports that were canceled by 8 pm CDT, according to tracking website flightaware.com, leaving thousands of travelers stranded.

"There's cascading delays because nothing can take off bound for Chicago from anywhere," said Doug Church, spokesman for the National Air Traffic Controllers Association, a union of air traffic controllers. "The impact is national and major."

At O'Hare, passengers were scrambling to find alternative transportation or bracing for long delays.

"I'm shocked at how calm everyone is. With everything going on in the world, maybe we're all managing our expectations. It's a fire in Aurora, it's not ISIS," said Cynthia Stemler of the Chicago suburb of Lake Bluff, who was heading to Newark, New Jersey, in a reference to the militant Islamic group at war in Syria and Iraq.

O'Hare is the main hub for United Airlines and a major hub for American Airlines. The airport averaged about 2,700 flights a day in August with a daily average of about 220,000 passengers passing through in the month, according data posted on its website.

Southwest Airlines Co suspended all flights through the day at Midway and Milwaukee's General Mitchell International Airport, the airline said in a statement.