"We are experiencing delays following an Aerodata issue impacting multiple airlines' ability to release flights".
At least five major airlines experienced major computer outages early Monday (April 1), causing massive travel headaches around the country. Flights from Atlanta Hartsfield-Jackson, New York's Newark Airport, and Dallas Fort-Worth were also substantially affected.
The airlines spent Monday morning apologizing to delayed customers around the country.
Southwest said it was a third-party vendor's computer outage that affected "data used in flight planning".
As of 8:23 a.m., travelers at O'Hare were facing delays of less than 15 minutes, while delays at Midway were averaging 22 minutes, according to the Chicago Department of Aviation.
Southwest said it lifted an internal ground stop at 7.05am ET, after about 40 minutes.
The glitch was later resolved, but lingering delays are expected as the airlines resume normal operations.
Southwest had 585 delays at 9:06 a.m.in NY, according to flight tracker website FlightAware.com, while JetBlue had 199.
Delta added: "I completely apologize, we are now experiencing a System-Wide Outage we are working diligently to get it back up and running". The airline did not anticipate flight cancellations.
American Airlines said the technical issue was related to AeroData, a company that makes aircraft location and balancing technology.