NEW YORK (AP) — Thousands of flights across the U.S. were canceled or delayed Wednesday after a system that offers safety information to pilots failed, and the government launched an investigation into the breakdown, which grounded some planes for hours.

The Federal Aviation Administration said preliminary indications “traced the outage to a damaged database file.” The agency said it would take steps to avoid another similar disruption.

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

Before a plane takes off, pilots and airline dispatchers must review the notices, which include details about bad weather, runway closures or other temporary factors that could affect the flight. The system was once telephone-based but moved online years ago.

The system broke down late Tuesday and was not fixed until midmorning Wednesday. The FAA took the rare step of preventing any planes from taking off for a time, and the cascading chaos led to more than 1,300 flight cancellations and 9,000 delays by early evening on the East Coast, according to flight-tracking website FlightAware.

Transportation Secretary Pete Buttigieg told a news conference that the problems “led to a ground stop because of the way safety information was moving through the system.”

After the failures were fixed, he warned that travelers could continue to see some effects “rippling through the system.”

Buttigieg said his agency would now try to learn why the system went down.

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

“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 FAA’s technology, and not just the NOTAM system.

Many of those systems “are old mainframe systems that are generally reliable, but they are out of date,” he said.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 the aviation industry has talked for years about trying to modernize the NOTAM system, but he did not know the age of the servers that the FAA uses.

“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. 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 air traffic picked up in the morning, the phone system was overwhelmed.

The FAA ordered all departing flights grounded early Wednesday, affecting all passenger and cargo flights. Some medical flights could get clearance, and the outage did not affect military operations.

President Joe Biden said that he was briefed by Buttigieg.

United Airlines temporarily delayed all U.S. flights before the ground stop.

While Southwest, who itself had issues just two weeks ago with flight cancellations, said it was monitoring the issues.

Buttigieg said the ground stop showed that “safety is going to be our North Star, as it always is.”

“We are now pivoting to focus on understanding the causes of the issue,” he said.

Pilots and safety officials have complained about NOTAMs for years, saying there are too many of them and that some are unnecessary and written in cryptic abbreviations.

The National Transportation Safety Board highlighted the large number of notices in its investigation of a near-disaster in 2017 in San Francisco. An Air Canada jet whose pilots had overlooked a NOTAM about a closed runway nearly landed by mistake on a parallel taxiway. They skimmed just over the tops of four other airliners waiting to take off.

The safety board’s chairman at the time, Robert Sumwalt, noted that the closed runway was mentioned on the eighth of 27 pages of notices for the San Francisco airport, and the entry was written in an arcane fashion.

“That’s what NOTAMs are. They are a bunch of garbage that no one pays any attention to,” he said.

The FAA said in a 2020 report that it modernized the distribution of the notices through a standardized digital format that was to be completed in July of that year.

As the extent of Wednesday’s breakdown became clear and airlines delayed more flights, passengers scrambled to rearrange trips. Many said they had trouble finding out how long the delays would last.

“There is just a lot of frustration, a lot of confusion,” said Ryan Ososki, who was trying to fly from Washington, D.C., to California for a conference.

Julia Macpherson was on a United flight from Sydney to Los Angeles when she learned of possible delays.

“As I was up in the air, I got news from my friend who was also traveling overseas that there was a power outage,” said Macpherson, who was returning to Jacksonville, Florida, from Tasmania.

Passengers at airports in Chicago, Atlanta and elsewhere reported similar experiences.

European flights into the U.S. appeared to be largely unaffected.

It was the latest headache for travelers in the U.S., who faced an even larger number of daily flight cancellations over the Christmas holiday due to a severe winter storm and a breakdown in crew-scheduling technology at Southwest Airlines.