The US Federal Aviation Administration lifted a nationwide ground stop on Friday ​after a communications failure at air traffic control centres caused chaos at airports and triggered the longest nationwide shutdown since the September 11, 2001, attacks.

Transportation Secretary Sean ‌Duffy said US airspace reopened at 8:15 pm Eastern Time (0115 GMT), but warned that departures would resume gradually and it would take time for airlines to restore normal operations.

“It’s going to be slow,” Duffy told reporters at the White House.

More than 6,600 flights were cancelled and 8,600 delayed, with another 1,500 already cancelled for Sunday, according to FlightRadar24. Aviation analysts said the four-hour nationwide grounding could disrupt airline schedules for days.

Duffy said tens of thousands of ​passengers were stranded at airports around the country. Duffy said he and Homeland Security Secretary Markwayne Mullin had met with President Donald Trump in the Oval Office to discuss the ​outage.

The failure is the latest and most dramatic sign of strain on a US air traffic control system beset by ageing technology, equipment and communications problems and a shortage of several thousand controllers.

Duffy said the trouble began at 3:36 pm, when air traffic control centres experienced instability in radar and communications systems.

“Our switches, our routers actually got overloaded ​with data, and they started to give us inconsistencies in the data from those routers, and because of that we shut the system down,” Duffy said, adding that at no stage did they completely lose radar ​or communications and they were able to safely land thousands of planes.

Duffy said the immediate problem had been fixed and that flying was safe, but warned that a broader overhaul of the ageing air traffic control system was needed to prevent similar failures.

$50 billion aviation overhaul

Duffy blamed Congress for failing to fund a new system that he said could prevent such outages, and called for $50 billion to modernise it.

Earlier on Friday, before the outage, the White House ​and Department of Transportation had urged Congress to approve $30.1 billion to upgrade the nation’s ageing airports and air traffic control infrastructure.

“What we have is an incredibly old air traffic control system,” Duffy ​said.

“Give us the money to fix it.”

Homeland Security Secretary Mullin said there was “zero evidence” so far that the outage resulted from a cyberattack or a rogue AI system, though investigators had not ruled out either possibility.

“It appears this ‌was mechanically driven, however, we’re still looking,” Mullin said.

The National Security Agency was working with the Department of Homeland Security on the investigation, he added.

The outage appeared to be affecting a telecommunications network overseen by L3Harris Technologies, according to two airline officials. The network handles critical radio frequencies used by air traffic controllers to communicate with pilots. L3Harris did not respond to a request for comment.

Pain not over for passengers

Though the ground stop was lifted, the disruption for passengers was far from over. From Boston to Seattle and New York to Los Angeles, passengers crowded departure halls and airline desks as delays and ​cancellations mounted and aircraft remained parked at gates.

Thousands ​of passengers faced being stranded overnight. Edward Kargbo, 50, from West Chester, Pennsylvania, had been scheduled to take a 9 p.m. Ethiopian Airlines flight to Lomé, Togo. Instead, he planned to spend the night at the airport before a rescheduled flight at 6 a.m. on Saturday.

“I have to squat the night here,” he said. “I’m stranded.”

Louise Maher, 47, was ​also caught up in the disruption at Newark after her British Airways flight to London was caught up in the aviation meltdown.

“I might ​cry at some point but not yet,” she said.

US airlines will not be required to provide compensation to stranded passengers because the carriers were not responsible for the FAA system failures.

The only other nationwide ground stop since the September 11 attacks came in January 2023, when the FAA halted domestic departures for about 90 minutes.

Investors were already pricing in a financial hit to airlines. Shares of American Airlines (AAL.O), opens new tab, United Airlines (UAL.O), opens new tab and Delta Air Lines (DAL.N), opens new tab fell between 1 percent and 2 percent ​in after-hours trading, while JetBlue Airways dropped about 3 percent.

American Airlines said it was prioritising flights to smaller cities outside its ​major hubs and international destinations to position aircraft and crews for a faster recovery.