NEW YORK — Thousands of flights across the U.S. were canceled or delayed Wednesday after a government system that offers safety and other information to pilots broke down, stranding some planes on the ground for hours.

The White House said there was no evidence that a cyberattack triggered the outage, which upended travel plans for millions of passengers. President Joe Biden said he directed the Department of Transportation to investigate.

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.

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 NOTAM system broke down late Tuesday and was not fixed until midmorning Wednesday, leading to more than 1,200 flight cancellations and more than 8,500 delays by early afternoon on the East Coast, according to flight-tracking website FlightAware.

Even after the Federal Aviation Administration lifted the order grounding planes, the chaos was expected to linger. 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, Los Angeles, New York and Atlanta saw 30% to 40% of flights delayed.

"There was a systems issue overnight that led to a ground stop because of the way safety information was moving through the system," Transportation Secretary Pete Buttigieg told a news conference. Although the problem was soon fixed, he warned that travelers could expect to see some effects "rippling through the system."

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

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 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 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.

"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 daytime traffic picked up, the phone system became overwhelmed.

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

Biden said that he was briefed by Buttigieg.

Buttigieg emphasized 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.

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. "I'm back to an hour and a half delayed (and) still unclear as to whether or not I should be boarding because I'd now miss my connection flight."

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 Florida from Hobart, Tasmania.

Once she lands in Los Angeles, she still has a connection in Denver on her way to Jacksonville, Florida.

She said there were no announcements on the flight about the FAA issue.

Similar stories came out of Chicago, Washington, Atlanta and other major U.S. airports.

European flights into the U.S. appeared to be largely unaffected. Carriers including Ireland's Aer Lingus and Germany's Lufthansa said their schedules were unaffected.

It was the latest headache for travelers in the U.S. who faced weather-related flight cancellations over the holidays and a broad breakdown at Southwest Airlines.

Passengers also ran into long lines, lost baggage, and cancellations and delays over the summer as travel demand roared back from the COVID-19 pandemic and ran into staffing cutbacks at airports and airlines in the U.S. and Europe.