The Federal Aviation Administration lifted a nationwide ground stop Friday evening after declaring it Friday afternoon. The stoppage delayed or cancelled more than 11,000 US flights, according to aviation tracker FlightAware. Transportation Secretary Sean Duffy said the airspace reopened at 8:15 p.m. Eastern Time. Duffy said the problem had been identified but did not specify what caused it. He acknowledged it would take time to clear the backlog of grounded aircraft.