An overnight system outage forced a halt to all U.S. departing flights. However, according to Reuters U.S. flights were slowly beginning to resume departures and a ground stop was lifted by the Federal Aviation Administration.

The cause of the problem had to do with a pilot-alerting system, which delayed thousands of flights in the United States. U.S. officials have since said they had so far found no evidence of a cyberattack.

“Normal air traffic operations are resuming gradually across the U.S. following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted. We continue to look into the cause of the initial problem,” the FAA Tweeted.

The number of flights impacted continued to steadily rise even after the ground stop was canceled. One issue airlines are facing is trying to get planes in and out of crowded gates, which is causing further delays.

According to reports, more than 5,400 flights had been delayed and 900 cancelled according to the FlightAware website as officials said it will take hours to recover from the halt to flights.