A nationwide US ground stop that caused thousands of flight cancellations was caused by an issue following a copper wire upgrade, Transport Secretary Sean Duffy said.

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 as a result of a cyber-attack, although an investigation is still under way, Duffy said.

Friday’s suspension was the longest such incident since the 11 September 2001 attacks on the US and the disruption is expected to last for days.

Maintenance was being carried out to switch from copper wire to fibre by a contractor called L3Harris, a technology provider company, Duffy said.

The US has “an incredibly old telecommunications system”, he added.

“It was such an old wire that it was being discontinued,” he said.

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 which led the government to shut down the airspace for safety reasons.

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.”

On Saturday L3Harris said 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,” it said in a statement.

“L3Harris takes the nationwide ground stop and its impact on travellers, airlines and air traffic operations seriously.”