Aircraft are seen at Ronald Reagan Washington National Airport in Arlington, Va, after the Federal Aviation Administration grounded flights nationwide. (AP Photo)A nationwide ground stop across the United States led to thousands of flight cancellations after an issue emerged following a copper wire upgrade, Transport Secretary Sean Duffy said.Duffy said that the government was trying to find out whether the problem stemmed from “incompetence” by the organisation doing the upgrade or faulty equipment.The US government does not believe a cyber-attack caused the nationwide ground stop; however, an investigation into the issue is still underway, said Duffy.The nationwide flight suspension on Friday was the longest such disruption since the September 11, 2001, attacks on the US, with the impact on air travel expected to continue for several days.Technology provider L3Harris was carrying out maintenance to replace copper wires with fibre, Duffy said.He said that the United States has “an incredibly old telecommunications system”.“It was such an old wire that it was being discontinued,” he said.Story continues below this adThe transport secretary explained that a router being worked on during a maintenance upgrade began flooding other air traffic control systems, disrupting communication with aircraft and prompting the US government to shut down the airspace for safety reasonsFederal Aviation Administration (FAA) chief Bryan Bedford said investigators had found no evidence of malicious activity or artificial intelligence involvement in the technical issue that triggered the nationwide flight ground stop.“We see zero evidence of anything nefarious, and zero evidence of anything AI-related,” Bedford said.L3Harris, on Saturday, had said that it completed over 70% of its work to modernise communications infrastructure before the shutdown happened.Story continues below this ad“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.”Flights across the continental US were grounded for more than four hours on Friday, with no departures permitted from 15:54 EST (3:54 PM). The disruption continued into Saturday, when more than 3,500 flights were cancelled and over 2,000 delayed, according to FlightAware.The Express Global Desk at indianexpress.com which delivers authoritative, verified, and context-dri... Read MoreStay updated with the latest - Click here to follow us on InstagramTags:US flight