More than 1,600 flights were grounded nationwide Friday following a fire at an Illinois air traffic control facility.

The disruption at the Federal Aviation Administration radar facility hit Chicago’s busy O’Hare and Midway airports the hardest, grounding virtually all flights at those airports for several hours.

A contract employee at a Chicago-area air traffic control center apparently set a fire and tried to kill himself, officials said Friday, shutting the facility and stopping all flights at the world’s second-busiest airport.

The airline tracking website FlightAware reported 971 O’Hare cancellations and 376 at Midway. Southwest Airlines said it has canceled all flights at Midway, as well as General Mitchell Airport in Milwaukee, until at least 7 p.m.

By midday, a few flights began taking off and landing again at O’Hare and Midway airports after a nearly five-hour gap. But the planes were moving at a much-reduced pace, officials said, and no one could be sure when full service would resume.