A nationwide US ground stop that caused thousands of flight cancellations and prompted travel chaos was caused by a technical issue following a copper wire upgrade, the transport secretary has said.
Sean Duffy told a news conference that the government was trying to find out whether the problem that led to air traffic controllers suspending take-offs for four hours on Friday had been caused by "incompetence" on the part of the company doing the upgrades or faulty equipment.
Though the ground stop was lifted on Friday evening, it had a knock-on effect on air travel on Saturday as airlines attempted to clear a backlog of flights.
Officials including Duffy had previously attributed the issue to outdated equipment.
Maintenance on US air traffic control systems was being carried out to switch from copper wire to fibre-optic cable by a contractor called L3Harris, a technology provider, Duffy said.
The US had "an incredibly old telecommunications system", he said, adding: "It was such an old wire that it was being discontinued."
The transport secretary explained that a router that was being worked on had started flooding other air traffic control systems and created communication issues with planes, prompting the shutdown on safety grounds.
The government was now trying to understand whether the root cause was the wire replacement or a fault with the router.
The four-hour suspension from 15:54 EST (19:54 GMT) on Friday was the longest such incident since the 11 September 2001 attacks on the US.
More than 15,000 flights in the US were delayed or cancelled that day, according to data from flight tracking sites, affecting an estimated 1.5 million passengers, with more than 5,900 cancelled and 7,000 delayed on Saturday.
In some cases, passengers were left stuck inside planes on the tarmac, unable to take off or to return to the gates. The ground stop also had a knock-on effect on schedules around the world, with some flights out of the UK and Europe cancelled.
The Federal Aviation Administration (FAA) said the US flight schedule should return to normal by the end of Sunday.
US officials have said they did not believe the issue was as a result of a cyber-attack or a rogue AI agent, though Duffy said an investigation into the exact cause was under way.
Bryan Bedford, who heads the FAA, told the same news conference: "We see zero evidence of anything nefarious and zero evidence of anything AI related."
L3Harris said on Saturday that it had completed more than 70% of the work it had been tasked to undertake to modernise US air traffic communications infrastructure before the shutdown.
The company said it was supporting the FAA in its investigation, adding: "L3Harris takes the nationwide ground stop and its impact on travellers, airlines and air traffic operations seriously."
EPA Passengers at Ronald Reagan Washington National Airport in Virginia stand in a sea of luggage.EPA
Duffy warned that a similar issue could arise again unless the US Congress approved upgrades to air traffic control infrastructure.
US President Donald Trump has requested $30.1bn (£22.8bn) for the upgrades. Both chambers of Congress are currently controlled by the Republican Party.
Duffy previously said the system needed a $50bn overhaul.
Former TSA Administrator John Pistole told the BBC that "the fact that they were able to get it restarted in four-and-a-half hours is good news - it shows a resilience in the system - but what's to say it's not going to happen tomorrow?"
A nationwide ground stop has only been ordered one other time since all flights were banned from take-off for several days following the 11 September 2001 attacks.
A failure in the Notice to Air Missions (Notam) system in 2023 prompted a grounding that lasted an hour and 52 minutes, delaying about 9,500 flights and cancelling about 1,300.
