L3Harris said on Saturday that it had completed over 70% of the work to modernise communications infrastructure before the shutdown. “We are supporting the FAA in its investigation to understand the conditions that led to the 13 November ground stop of our nation’s air travel,” the company said in a statement. “L3Harris takes the nationwide ground stop and its impact on travellers, airlines and air traffic operations seriously.”

Bryan Bedford, who heads the Federal Aviation Administration (FAA), told the same news conference: “We see zero evidence of anything nefarious, and zero evidence of anything AI related.”

No flights were allowed to take off from the continental US for more than four hours from 15:54 EST (19:54 GMT) on Friday, the longest such incident since the 11 September 2001 attacks on the US. 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. Disruption from the shutdown is expected to last for days.

Transport Secretary Sean Duffy told a news conference that the government was trying to find out whether the problem was caused by “incompetence” on the part of the company doing the upgrades, or faulty equipment. The US government does not believe it was the result of a cyber-attack, although an investigation is still under way, he said.

Maintenance was being carried out to switch from copper wire to fibre by a contractor called L3Harris, a technology provider company, Duffy said. He explained that a router being worked on had started flooding other air traffic control systems and created communication issues with planes, which led the government to shut down the airspace for safety reasons.

Duffy said the US has “an incredibly old telecommunications system”, adding of the cable involved: “It was such an old wire that it was being discontinued.”

The knock-on effects continued into Saturday. More than 3,500 flights were cancelled and more than 2,000 delayed in the US on Saturday, according to tracking website Flight Aware. In some cases, passengers were left stuck inside planes on the tarmac, unable to take off or to return to the gates. Some international flights were allowed to depart US airports, in an apparent effort to clear space.

Duffy warned that a similar issue could arise again unless the US Congress approved upgrades to air traffic control infrastructure. President Donald Trump has requested $30.1bn for the upgrades. Both chambers of Congress are currently controlled by the Republican Party.

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?”