United Airlines flight schedules are gradually returning to normal after a nationwide technical issue forced flight cancellations and delays Wednesday night. Residual delays and cancellations continued Thursday morning for United Airlines flyers at Chicago’s O’Hare International Airport after one of the airline’s computer systems went down Wednesday night,

United Airlines resolved the system disruption that delayed over 1,000 flights, but residual delays are expected and the cause is unknown.

The United ground stop has been lifted at airports in multiple cities as the airline resolved a technical issue. There are still residual delays.