The rare stop, which lasted about four hours, grounded thousands of flights at all U.S. airports in one of the biggest disruptions of the nation's air travel system since the September 11th attacks. 

In a Friday night news conference, Transportation Secretary Sean Duffy said that air traffic control facilities had experienced "intermittent" outages of their radar and communications systems beginning earlier in the afternoon. Duffy blamed outdated technology for the failures. 

"We have old switches, old routers. We use 1980s little copper lines. It's an old system that went down and went down everywhere. It got overloaded," he said.

But when pressed by reporters on exactly why those systems would become overwhelmed on an otherwise normal travel day, Duffy did not have a ready answer. 

Homeland Security Secretary Markwayne Mullin joined Duffy in the White House briefing room and said there was no evidence the issue was due to an attack.

"We have seen zero evidence this is a cyber attack," he said.

A flight board showing delayed flights is pictured at Ronald Reagan Washington National Airport, Friday.

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As of 9 p.m. ET, more than 8,000 flights had been delayed Friday and 7,000 had been canceled in the U.S., according to FlightAware, a flight tracking site. The turmoil comes on the heels of nearly 7,000 flight delays on Thursday.

The FAA said the issue affected Air Route Traffic Control Centers — the regional air traffic control centers across the country that handle high-altitude traffic.

FAA leaders did not brief the media at the White House press conference, and did not provide any further detail on the communication outages that led to the ground stop.

This was only the second nationwide ground stop since September 11, 2001. The U.S. last issued a ground stop of this scale in 2023, when all U.S. airports were shut down due to an outage in the NOTAM (Notice to Air Missions) system, which alerts pilots and airports of real-time hazards.

That outage prompted fears of a cyberattack, but the FAA later determined the issue was caused when contract workers inadvertently deleted critical files.

Senator Richard Blumenthal (D-CT) released a statement late Friday evening calling for a "prompt, penetrating investigation" into the causes of Friday's ground stop.

"A widespread communication outage of this scale is highly alarming — a clear danger to national security, as well as safe, convenient travel close to the holidays. The Department of Transportation owes Americans credible, accurate action and factual disclosure," the statement read.

The FAA has recently begun rolling out an AI-enhanced software tool known as the SMART system as part of its ambitious overhaul of the nation's air traffic control system. But Air Space Intelligence, the company that makes the tool, quickly dismissed speculation that its product could be connected to the flight disruptions.

"The current ground stop is not based on or a result of the SMART system, any product of ASI or our integration with FAA and ATC," the company said on social media.

Passengers stranded

The nationwide ground stop caused chaos in airports and headaches for travelers — many of whom are already too familiar with flight delays and cancellations during the holiday season.

At the Friday night press conference, Duffy said flights would resume but would take time to clear.

At Dallas Fort Worth International Airport, Regina Harling said she waited hours — long past when her flight was supposed to depart — before she learned of the nationwide grounding by checking the news on her phone. 

"There's still a sign right in front of me that says we're boarding in 16 minutes," she said Friday afternoon. 

An airline worker shows people how to apply for a refund at LaGuardia airport on Friday.

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Harling needed to travel last minute to San Diego for a family emergency. Stuck at the airport, she said she was thinking about how soon she could make it to her family, but she's not optimistic.

"I was just thinking to myself that it always feels like it's getting harder to travel than it used to be," she said.

At Chicago O'Hare International Airport, Giliann Karon was on board a plane when the ground stop was ordered. Karon said she had already experienced a flight delay the day before due to bad weather in New York City, where she is heading.

"It's just kind of like, 'Oh, of course this is happening,'" she said.

Deepa Shivaram contributed to this report.