CHICAGO (NewsNation) — Thousands of flights were delayed or canceled across the U.S. Wednesday after air traffic was grounded for hours due to a government computer system failure.

The Federal Aviation Administration (FAA) announced an outage of its computer system called the Notice to Air Missions, or NOTAM. The NOTAM system is responsible for communicating flight hazards and real-time restrictions to pilots, making it hazardous for pilots to take off without it.

The NOTAM system broke down late Tuesday. The outage caused a temporary nationwide pause in flight operations early Wednesday morning. The FAA has since announced that normal air traffic operations have resumed and lifted the ground stop order, but travel disruptions are expected to grow as flight backups compound.

“Safety is always our first priority, and ensuring flight safety was the reason for this morning’s ground stop while the affected systems were restored and checked,” Secretary of Transportation Pete Buttigieg said on Twitter. “As normal flight operations have resumed, FAA continues to assess the causes of the outage.”

Buttigieg later tweeted that there was no evidence of a cyberattack, and a preliminary investigation traced the issue back to a “damaged database file.”

But Greg Feith, a former senior air safety investigator with the National Transportation Safety Board, said “it may be a little premature” to make any determination, given recent attacks on electrical grids.

He joined “Rush Hour” on Wednesday to discuss the malfunction of the software, which many experts have noted is outdated. You can watch Feith’s full interview here.

“We won’t really know (the cause) until they get into the investigative mode. Right now, they are talking about cyberattacks — that’s always a possibility and I think that’s still on the table,” Feith said. “They’re going to have to do a very in-depth study of not only the software but the hardware and the vulnerabilities of that system.”

The administration originally ordered all airlines to pause all domestic departures until 9 a.m. ET to allow the agency to validate the integrity of flight and safety information.

However, travel journalist Peter Greenberg told NewsNation that the 9 a.m. ET estimated time of recovery was just a relative term.

He said, “The airlines will not get back on track for many hours today. You’ll see a number of delays morphed into full cancellations. Otherwise, the airlines can’t stabilize their schedules.”

As of 6 p.m. ET, more than 9,000 flights have been delayed within the U.S., and more than 1,200 flights have been canceled, according to flight-tracking website FlightAware.

Delay times averaged around an hour and a half or more at some of the nation’s busiest airports, according to the website.

Greenberg said the good news for travelers affected by the pause is that it isn’t a peak travel time — in fact, it’s actually a weak travel time. This means that if travelers run into cancellations, the odds are that they may be able to get on another flight later in the day, he said.

For departing flights, the pause started a cascading ripple effect that Greenberg said could easily extend into Thursday.

Buttigieg briefed President Joe Biden on the FAA system outage Wednesday morning, according to White House press secretary Karine Jean-Pierre.

“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 tweeted.

The cause of the outage is still undetermined, but Greenberg said this was the first time he’s seen a ground stop since the historical events on Sept. 11, 2001.

“Aircraft can still land safely, just not take off right now. They don’t know what the cause of it is, they expect in a couple of hours they’ll have a good sense of what caused it and will respond at that time,” Biden said.

The FAA supported Biden’s statement, saying that all flights in the sky during the departure pause were safe to land. The administration explained that pilots check the NOTAM system before they fly.

“A Notice to Air Missions alerts pilots about closed runways, equipment outages and other potential hazards along a flight route or at a location that could affect the flight,” the FAA tweeted.

Greenberg said on “Morning in America” that currently, NOTAM is an antiquated system, but the FAA is trying to change that. NextGen, also known as the Next Generation Air Transportation System, is a multibillion-dollar infrastructure program the FAA is working on to modernize the U.S. National Airspace System.

“The irony about it is that the FAA has been talking about NextGen for two generations. This will be a huge wake-up call to get Congress to act to perhaps possibly give them more money to get it in place sooner,” Greenberg said.

“United has temporarily delayed all domestic flights and will issue an update when we learn more from the FAA,” United Airlines told NewsNation in an initial statement.

Once the ground stop lifted, United announced they have activated a travel waiver for any customers who need to change their plans, including offering refunds for flyers who no longer want to travel.

The U.S. Travel Association released a statement, calling for federal action in response to the outage:

“Today’s FAA catastrophic system failure is a clear sign that America’s transportation network desperately needs significant upgrades. Americans deserve an end-to-end travel experience that is seamless and secure. … We call on federal policymakers to modernize our vital air travel infrastructure to ensure our systems are able to meet demand safely and efficiently.”

The Associated Press contributed to this report.