NEW YORK (PIX11) — The Federal Aviation Administration ordered all domestic flight departures held Wednesday morning amid a significant computer outage that brought air traffic to a grinding halt nationwide.

The massive ground stop was lifted shortly before 9 a.m., but airports still faced a backlog of thousands of flights that had accumulated over hours of downtime early Wednesday.

“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 FAA tweeted at 8:50 a.m., about 90 minutes after the agency had ordered all domestic departures held until 9 a.m. “The ground stop has been lifted.”

But even as operations were cleared to resume, there were more than 3,700 delayed flights within, into, or out of the United States, according to the flight tracking website FlightAware. More than 640 other flights had been cancelled.

The sweeping wave of delays and cancellations came in the wake of the FAA’s Notice to Air Missions System going down early Wednesday. The system, also known as NOTAM, keeps pilots in the know about everything from minor airport construction to potentially hazardous flight conditions. All aircraft, including commercial and military flights are required to use the system, underscoring the potential for widespread upheaval during an outage.

“The FAA is working to restore its Notice to Air Missions System,” the agency wrote in an initial 6:29 a.m. tweet. “We are performing final validation checks and reloading the system now. Operations across the National Airspace System are affected.”

In another FAA tweet sent at 6:57 a.m., the agency confirmed the “outage,” adding that “some functions are beginning to come back on line” but that the restoration was ongoing, leaving operations “limited.”

In the 8:50 a.m. tweet, the FAA said that the agency continues “to look into the cause of the initial problem.”

White House Press Secretary Karine Jean-Pierre tweeted that President Biden had been briefed on the situation by Secretary of Transportation Pete Buttigieg, adding that there was no evidence to immediately suggest a cyberattack.

“There is no evidence of a cyberattack at this point, but the President directed DOT to conduct a full investigation into the causes,” Jean-Pierre wrote in part.