Departures from every airport in the United States were halted for more than four hours on Friday, November 13, after radar and communications data at air traffic control centers became unreliable. More than 3,500 planes were in the air when the stop began, Transportation Secretary Sean Duffy said, and the tracking service FlightAware counted nearly 7,000 delayed or canceled flights. Three days earlier, the Federal Aviation Administration had said it was extending SMART, an artificial intelligence program that suggests routes and schedules to help flights avoid storms and congestion, to all 22 centers that guide planes between airports. Homeland Security Secretary Markwayne Mullin said earlier on Friday that agencies were checking for a link.

By that night the government had ruled it out. Duffy said SMART is not connected to the radar and flight-tracking systems controllers rely on, and that there was “no possible way” it affected the outage. Its maker, Air Space Intelligence, which holds a 12-year, $875 million FAA contract, also denied any role. Officials blamed old switches and routers that were overloaded with data, in a network Duffy said still relies on copper lines from the 1980s. Mullin said the overload appeared to begin as the agency added new equipment and software patches, and that nothing was completely certain yet. FAA technicians are still investigating. Duffy asked Congress for $50 billion to rebuild the network, and travelers were told some flights would not leave until Saturday morning.