Flights across the U.S. slowly resumed Wednesday after a nationwide ground stop by the Federal Aviation Administration, stemming from the outage of a crucial piece of technology.

The failure of the Notice to Air Missions system, or NOTAM, caused airlines to cancel more than 1,300 flights, and delay nearly 10,000 more, according to flight tracker FlightAware.com.

The FAA said that early investigative work traced the blackout to a "damaged database file," but the agency is still working to determine the root cause.

"At this time, there is no evidence of a cyberattack. The FAA is working diligently to further pinpoint the causes of this issue and take all needed steps to prevent this kind of disruption from happening again."

This NOTAM technology, which alerts pilots and airports of real-time hazards, failed by late Tuesday. Pilots could still get updates through other means while there were fewer flight departures. But as morning approached on the East Coast — and more flights were slated to take off — the issue hadn't been resolved. The FAA ordered a ground stop a little after 7 ET and ended it around 9. It took hours for flights to get back on track.

Transportation Secretary Pete Buttigieg emphasized that the shutdown was needed to keep passengers safe. He also "directed an after-action process to determine root causes and recommend next steps."

This incident comes just weeks after major disruptions with Southwest Airlines over the holidays led to more than 16,700 canceled flights.

Similar disruptions arose in Canada on the same day. NAV CANADA, which owns and operates that country's civil air navigation system, reported its own issues with their NOTAM technology and subsequent delays.

"NAV CANADA continues to investigate the cause of the outage; at this time, we do not believe it to be related to the FAA outage experienced earlier today,
the company said late Wednesday.

The outage raises concerns

Experts told NPR that the NOTAM system has never gone down like it did on Wednesday, and the last time there was a nationwide ground stop of departing planes in the U.S. was during the 9/11 attacks.

Mike McCormick, a former safety official at the FAA and an assistant professor at the Embry Riddle Aeronautical University, said the incident raises concerns as the FAA has been investing millions of dollars into updating its systems.

"The surprising part to me that after years of upgrade and investment in the next generation aviation system, how one — whatever it may be — problem caused this complete failure in the system. And there should never be a single point of failure," he said.

What passengers can do now

The updates from federal authorities likely offered little comfort to passengers whose travel plans were thrown for a loop. Some travelers had to rebook, deal with delays or scrap their original plans altogether.

In New York City, Benjamin Fashola told Gothamist he had a frustrating morning at LaGuardia Airport. His original flight was canceled, then re-booked, and subsequently delayed.

"It's very frustrating," he said. "Patience is a tool we're going to use right now because at the end that's all we can do is be patient."

Unlike with the Southwest Airlines cancellations over the holidays, it's unclear who can be responsible for covering costs tied to delays and cancellations when the problem is not an airline issue.

A few airlines are giving travelers some flexibility in the meantime.

United is giving "a travel waiver for any customers who need to change their plans, including offering refunds for customers who no longer want to travel," the company announced. Customers who want to get a refund can submit a request through united.com/refund.

All Southwest customers can "rebook in the original class of service or travel standby (within 14 days of the original date of travel between the original city-pairs and in accordance with our accommodation procedures) without paying additional charges."

Lawmakers and industry players want answers

Buttigieg spent much of the day addressing questions about how this happened and defending the ground stop.

"My top priority right now ... is to understand the root cause, understand how it could have led to this level of disruption, and understand how to make sure it doesn't happen again," he said after flight departures were allowed to resume.

The issues with NOTAM were resolved in a matter of hours, but lawmakers from both political parties as well as industry stakeholders are raising questions about the state of the technology that the air transportation system relies upon.

Sen. Maria Cantwell, a Democrat and the chair of the Senate Commerce, Science and Transportation Committee, said the panel will look into the cause and how to prevent future outages as it prepares for "FAA reauthorization legislation."

Republican Rep. Sam Graves, the new chair of the House transportation committee and a pilot, issued a strong statement against the FAA, saying this outage "highlights a huge vulnerability in our air transportation system."

Graves said he will ask the FAA to provide a full briefing to Congress as soon as the agency learns more.

"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."

Geoff Freeman, president and CEO the U.S. Travel Association, said the system failure is a sign that the country's transportation network needs major upgrades. The association is an industry group representing "all components of the travel industry."

"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," he said in a statement. "We call on federal policymakers to modernize our vital air travel infrastructure to ensure our systems are able to meet demand safely and efficiently."