AFP.com, 11 Jan, 2023- US flights were slowly beginning to resume departures and a ground stop was lifted after the Federal Aviation Administration (FAA) scrambled to fix a system outage overnight that forced a halt to all US departing flights.

“Normal air traffic operations are resuming gradually across the US 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 said in a Tweet. The FAA had earlier ordered airlines to pause all domestic departures after its pilot alerting system crashed.

The FAA said shortly before 8:30 a.m. departures were resuming at Newark and Atlanta airports “due to air traffic congestion in those areas. We expect departures to resume at other airports at 9 a.m. ET.”

The FAA is expected to implement a ground delay program in order to address the backlog of flights halted for hours. Flights already in the air were allowed to continue to their destinations. US President Joe Biden ordered the Transportation Department to investigate the outage and said the cause of the failure was unknown at this time. Asked if a cyber attack was behind the outage, Biden told reporters at the White House, “We don’t know.”

The FAA said it was working to restore a system that alerts pilots to hazards and changes to airport facilities and procedures that had stopped processing updated information.

“The FAA is still working to fully restore the Notice to Air Missions (NOTAM) system following an outage. While some functions are beginning to come back on line, National Airspace System operations remain limited,” the FAA said on Twitter.

A total of 3,704 US flights were delayed as of 8:39 a.m. ET, flight tracking website FlightAware showed. Another 656 were canceled as of 8:39 a.m.