A system outage at the Federal Aviation Administration halted air traffic across the country Wednesday.

Nearly 11,000 flights were delayed within, into or out of the United States, according to the flight tracking website FlightAware. More than 1,300 U.S. flights were canceled.

The breakdown showed how much American air travel depends on an antiquated computer system that generates alerts called NOTAMs — or Notice to Air Missions — to pilots and others.

The FAA said preliminary indications “traced the outage to a damaged database file." The agency said it would take steps to avoid another similar disruption.

Before a flight takes off, pilots and airline dispatchers must review the notices, which include details about weather, runway closures or construction and other information that could affect the flight. The system was once telephone-based, with pilots calling dedicated flight service stations for the information, but it has moved online.

The FAA had grounded all domestic flights until 9 a.m. Eastern time, but it announced shortly before then that the ground stop was lifted. 

The ground order was implemented to allow the agency time to "validate the integrity of flight and safety information," the FAA said. 

But even after the order was lifted, chaos continued throughout the day, with delays and cancellations being reported as the day went on. More than 21,000 flights were scheduled to take off Wednesday in the U.S., mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Airports in Chicago, New York and Atlanta saw 40% to 53% of flights delayed.

Transportation Secretary Pete Buttigieg wrote in a Twitter post that the "FAA has determined that the safety system affected by the overnight outage is fully restored." 

"Our number one priority is safety," Buttigieg told MSNBC in an interview on Wednesday. "It's one of the reasons why today, for example, while this issue was being worked out on the system side, there was this ground stop of about an hour and a half nationwide to make absolutely sure, out of an abundance of caution, that no aircraft could take off without the necessary safety information, and we're always going to err on the side of safety."

"Having said that, we also have to make sure we understand everything there is to understand about this situation, so that we can assure that a disruption like that doesn't happen again," Buttigieg conceded. "Because on a system that has to operate 24/7 – and that's how the national aviation system works – even if a primary system is down for some reason, there needs to be the kind of redundancy that you can flexibly deal with at all times and keep things running."

White House press secretary Karine Jean-Pierre said Wednesday morning that "there is no evidence of a cyberattack at this point," though President Biden directed the Transportation Department "to conduct a full investigation into the causes" of the outage.

Biden told reporters Wednesday morning that while they didn't know then the cause of the outage, planes in the air could still land safely.

"I just spoke with Buttigieg," Biden said. "They don't know what the cause is. I told them to report directly to me when they find out."

When asked if the outage highlighted a critical vulnerability in the system, Buttigieg told MSNBC that "part of what you're seeing here is an abundance of caution and making absolutely certain that operations were safe."

"This is one of the reasons why we need to get these answers and have this review of exactly what happened," he continued. "Because we need to understand whether this reflects a systemic issue and what would be required so there's no single point of failure here."

"There need to be redundancies and layers and layers of protection here," Buttigieg added. "This is an incredibly complex system. 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."

When asked if the outage was the result of foul play by a foreign or domestic source, Buttigieg said '"there hasn't been any indication of that," but would not fully rule that possibility out.

"FBI has spoken to this and, of course, FAA is looking closely at that as well," Buttigieg said, adding: "There is no direct indication of any kind of external or nefarious activity, but we're not yet prepared to rule that out."

In a statement on Wednesday, Missouri Rep. Sam Graves, the new chairman of the House Transportation and Infrastructure Committee, demanded answers about what he called the FAA's "system meltdown."

Graves said that the outage to the NOTAM system "highlights a huge vulnerability in our air transportation system" and drew comparisons to the Southwest Airlines cancellations last month.

"Just as Southwest’s widespread disruption just a few weeks ago was inexcusable, so too is the DOT’s and FAA’s failure to properly maintain and operate the air traffic control system," Graves wrote.

"This incident also underscores the number of empty desks and vacant offices at the FAA. Centuries of combined experience has gone out the door in the past several years and far too few of these positions have been filled," Graves wrote. "The FAA does not run on autopilot – it needs skilled, dedicated, and permanent leadership in positions across the agency, starting with the Administrator’s office."

The FAA is currently being led by acting Administrator Billy Nolen, previously the agency's associate administrator for aviation safety. Biden in July nominated Phillip Washington, currently the CEO of Denver International Airport, to serve as the agency's administrator, but he has not yet received a confirmation hearing.

“I have many questions about what transpired today, and I expect the FAA to provide a full briefing to Members of Congress as soon as they learn more," Graves concluded. "I will also be leading an oversight letter with my colleagues to make sure that we know what went wrong, who’s responsible, and how this is going to be prevented in the future. And just as DOT expected Southwest to make passengers whole after their leadership failures, I expect a prompt update on DOT’s efforts to do right by the passengers it has wronged.”  

NOTAM is a critical tool that provides pilots with essential information that can impact the safety of flights, from potential for thunderstorms to icing, and even runway construction.

"There are other sources of information that you can get this, but for safety reasons, this is one of the critical systems," Professor Laurie Garrow, co-director of the Center for Urban and Regional Air Mobility at Georgia Tech, told Spectrum News' Angi Gonzalez. "So you may be able to take off a certain number of flights without it, but you're not going to want to run systemwide without this information."

Longtime aviation insiders could not recall an outage of such magnitude caused by a technology breakdown. Some compared it to the nationwide shutdown of airspace after the Sept. 11 terror attacks in 2001.

“Periodically there have been local issues here or there, but this is pretty significant historically,” said Tim Campbell, a former senior vice president of air operations at American Airlines and now a consultant in Minneapolis.

Campbell said there has long been concern about the Federal Aviation Administration’s technology, and not just the NOTAM system.

“So much of their systems are old mainframe systems that are generally reliable but they are out of date,” he said.

John Cox, a former airline pilot and aviation safety expert, said there has been talk in the aviation industry for years about trying to modernize the NOTAM system, but he did not know the age of the servers that the FAA uses.

He couldn’t say whether a cyberattack was possible.

“I’ve been flying 53 years. I’ve never heard the system go down like this,” Cox said. “So something unusual happened.”

According to FAA advisories, the NOTAM system failed at 8:28 p.m. Eastern on Tuesday preventing new or amended notices from being distributed to pilots. The FAA resorted to a telephone hotline to keep departures flying overnight, but as daytime traffic picked up it overwhelmed the telephone backup system.

"There would be a series of tests that the FAA would have gone through this morning to make sure that that system not only looked like your computer screen was back up and running, but that it was behaving as you expected, communicating information correctly," Garrow told Spectrum News.

The FAA ordered all departing flights grounded early Wednesday morning, affecting all passenger and shipping flights.

Some medical flights could get clearance and the outage did not impact any military operations or mobility.

Flights for the U.S. military’s Air Mobility Command, were not affected.