Flights in the United States have resumed after the Federal Aviation Administration issued a nationwide ground stop, only the second since the September 11, 2001 attacks. Transportation Secretary Sean Duffy blamed a communications issue affecting 22 air traffic control centres, and FlightAware reported about 10,500 delayed and more than 1,800 cancelled flights, while Homeland Security Secretary Markwayne Mullin said there was no indication of a nefarious act but investigators would examine any link to a new AI system.