Flights across the U.S. resumed Wednesday morning, several hours after the Federal Aviation Administration suffered a computer outage that forced it to halt all departures nationwide while it scrambled to resolve the issue.
The FAA said the crippling delays that affected thousands of flights appear to have been caused by a problem in the Notice to Air Missions system, or NOTAM, which sends pilots vital information they need to fly.
A corrupted file affected both the primary and the backup systems, a senior government official said Wednesday evening, adding that officials continue to investigate.
"The FAA is continuing a thorough review to determine the root cause of the Notice to Air Missions (NOTAM) system outage," the agency said in a statement. "Our preliminary work has traced the outage to a damaged database file. At this time, there is no evidence of a cyber attack."
Transportation Secretary Pete Buttigieg said Wednesday evening that the FAA will further pinpoint the source and identify steps to prevent it from happening again, and he echoed the agency in saying there is no evidence of a cyber attack.
"One of the questions we need to look at right now, and one of the things I’m asking from FAA, is what’s the state of the art in this form of message traffic?" Buttigieg told NBC News' Andrea Mitchell earlier Wednesday. "And again, how is it possible for there to be this level of disruption?”
President Joe Biden ordered an investigation after he was briefed by Buttigieg.
Sen. Maria Cantwell, D-Wash., who heads the Commerce Committee, which oversees the FAA, said it, too, will look into the matter.
“The No. 1 priority is safety," Cantwell said in a statement. "As the Committee prepares for FAA reauthorization legislation, we will be looking into what caused this outage and how redundancy plays a role in preventing future outages. The public needs a resilient air transportation system.”
Meanwhile, the top Republican on the committee, Ted Cruz of Texas, said the "FAA’s inability to keep an important safety system up and running is completely unacceptable and just the latest example of dysfunction within the Department of Transportation."
The delays came just weeks after Southwest Airlines caused travel chaos by canceling more than 2,500 of its flights during the Christmas season.
Buttigieg acknowledged the frustrations of travelers in his interview with Mitchell but said safety always comes first.
“This is an incredibly complex system,” Buttigieg said. “So glitches or complications happen all the time. But we can’t allow them to ever lead to this level of disruption, and we won’t ever allow them to lead to a safety problem.”
The FAA lifted the ground stop around 8:50 a.m., and normal air traffic operations began resuming gradually. But by then airports across the country were already crowded with frustrated travelers and a backlog of flights.
As of noon, more than 7,300 flights within, to and out of the U.S. had been delayed, according to the online flight tracker FlightAware. More than 1,100 flights were listed as canceled.
Passengers stuck at O'Hare International Airport in Chicago greeted the latest air travel disruption with a collective shrug.
"It kind of is what it is," Teresa Ziakas said. "I would rather be safe and sound than worry about being delayed by a couple hours."
Traveler Martin Johansen echoed Ziakas.
"Planes get delayed," Johansen said as he waited with his wife, Barbara. "That’s just the way life is, so you get used to it."
Any person whose flight was canceled is entitled to a full refund, Transportation Department guidelines say. And major domestic carriers like American Airlines, United Airlines and Delta said they would waive the fees of travelers who want to rebook flights.
The first sign that it was likely to be a massive incident came around 7:20 a.m., when the FAA sent out a tweet ordering the airlines to pause all domestic departures until 9 a.m. ET “to allow the agency to validate the integrity of flight and safety information" as it worked to restore the NOTAM system.
All flights already in the sky were safe to land, the FAA said.
“Pilots check the NOTAM system before they fly," the agency said. "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."
As the drama unfolded, cybersecurity experts said the likeliest cause was a bad software update.
“Today’s FAA catastrophic system failure is a clear sign that America’s transportation network desperately needs significant upgrades," U.S. Travel Association President and CEO Geoff Freeman said in a statement. "Americans deserve an end-to-end travel experience that is seamless and secure. And our nation’s economy depends on a best-in-class air travel system.
"We call on federal policymakers to modernize our vital air travel infrastructure to ensure our systems are able to meet demand safely and efficiently," he said.
Buttigieg tweeted that he had "been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots."
United Airlines said earlier that it had temporarily delayed all domestic flights, while Southwest Airlines said it was "closely monitoring" the situation but warned customers to brace themselves for travel troubles.
Soon, the major airports were doing the same.
"An FAA system outage is causing ground stops at AUS and other airports across the country," Austin-Bergstrom International Airport in Texas tweeted.
"Arriving & departing passengers can expect delays this morning & through the day," it said, adding: "Please stay in contact with your airline & check your flight status before heading to AUS."
A number of social media users said they had been affected.
Heather Allen, 32, was meant to fly from New York City to Seattle with her fiancé to visit her family for a delayed holiday visit. She was watching a movie on her plane, still on the tarmac at John F. Kennedy International Airport, when she and other passengers were told to get off their Delta Airlines flight.
She said she had been on the plane for about an hour before she had to get off the plane and that she learned of the outage by reading the news on Twitter.
“Trying to be patient, but feeling frustrated,” Allen said. She said the situation at the airport was “not currently chaotic, but could be if delays are longer.”
The flight delays in the U.S. had a ripple effect on some airports abroad. The international airport operator Aéroports de Paris, or Airports of Paris, said all flights by U.S. carriers had been delayed. It said non-U.S. airlines were flying out as normal without interruption.
Air France said all of its U.S.-bound flights were not affected by the FAA computer outage and were operating as planned. It said it continued to monitor the situation.
A spokesperson for Gatwick Airport in London said, "As far as we are aware, we are still operating to/from the U.S. at the moment."