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 suspens...