The United States Federal Aviation Administration (FAA) lifted a nationwide ground stop on Friday after a communications failure at air traffic control centres caused chaos ​at US airports and triggered the longest shutdown since the September 11, 2001, attacks. Transportation Secretary Sean Duffy said US airspace reopened at 8:15pm Eastern Time (6:15am PKT the next day), but warned that departures would resume gradually and that airlines would need time to restore normal operations. "It's going to be slow," Duffy told reporters at the White House. By 11:30pm, nearly 10,000 flights had been cancelled, including more than 2,200 each at Delta Air Lines and American Airlines, and 1,800 at United Airlines, according to Flightradar24. The plane-tracking website said some aircraft at major airports had waited more than five hours for a gate. The four-hour nationwide grounding could ​disrupt airline schedules for days, aviation analysts said. Duffy said tens of thousands of passengers were stranded at airports nationwide. Duffy said he and Homeland Security Secretary Markwayne Mullin had ​met with President Donald Trump in the Oval Office to discuss the outage. Read: EU flights hit by wave of bomb hoaxes 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 3:36pm, 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 not funding 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. Read more: Trump's AI task force met Thursday, discussed AI-powered cyberattacks "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 9pm Ethiopian Airlines flight to Lomé, Togo. Instead, he planned to spend the night at the airport before a rescheduled flight at 6am 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 ​disrupted by 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, United and Delta ⁠fell between 1% ​and 2% in after-hours trading, while JetBlue Airways dropped about 3%. American Airlines said it was prioritising flights to international destinations ​and to smaller cities outside its major hubs to position aircraft and crews for a faster recovery.