NEW YORK (AP) — Planes were stuck on the ground for hours across the United States on Wednesday, leading to thousands of canceled and delayed flights after a government system used to give pilots safety and other information broke down overnight.

The White House initially said that there was no evidence of a cyberattack behind the outage that ruined travel plans for millions of passengers. President Joe Biden said Wednesday morning that he directed the Department of Transportation to investigate.

The outage showed how dependent the world’s largest economy is on air travel, and how much air travel depends on an antiquated computer system to generate 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 information 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 has moved online.

The NOTAM system broke down late Tuesday and was not fixed until 9 a.m. Eastern on Wednesday, leading to about 1,200 flight cancelations and more than 7,800 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 in the U.S. Wednesday, 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 were seeing between 30% and 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 said at a press conference. “That was resolved, which allowed the ground stop to be lifted at 9 this morning, but through the day we’re going to see the effects of that rippling through the system.”

Buttigieg said his agency was now turning to understand what caused the NOTAM system to go 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 terror attacks of September 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 FAA’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.

“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.

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.

Biden said Wednesday morning that he was briefed by Buttigieg.

“I just spoke to Buttigieg. They don’t know what the cause is. But I was on the phone with him about 10 minutes,” Biden said. “I told him to report directly to me when they find out.

Buttigieg acknowledged the flight delays and cancellations caused by the outage, but emphasized “the main thing I want everybody to understand is that every step of the way 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 information about 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 work 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 Airlines flight from Sydney to Los Angeles on Wednesday 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 flight to Jacksonville, Florida.

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

Macpherson said she had already experienced a delay in her travels because her original flight from Melbourne to San Francisco was canceled and she rebooked a flight from Sydney to Los Angeles.

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 from Ireland’s Aer Lingus to Germany’s Lufthansa said there was no impact on their schedules.

It was the latest headache for travelers in the U.S. who faced flight cancelations over the holidays amid winter storms and a breakdown with staffing technology at Southwest Airlines. They also ran into long lines, lost baggage, and cancelations 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.