The Federal Aviation Administration said it will begin clearing flights to depart in coordination with its command center as en route facilities recover, while keeping its nationwide ground stop in effect after simultaneous radar and radio frequency outages hit all 22 Air Route Traffic Control Centers. An airline source told CNN after a crisis call that the agency still had no clear answer on a cause and that the failure was “affecting different facilities in different ways.” CBS News reported that westbound flights from West Coast airports, including routes to and from Hawaii, and some eastbound transatlantic flights were approved to depart.

Air Space Intelligence, the developer of the $875 million SMART artificial intelligence routing tool the FAA expanded across all 22 centers earlier this week, said the ground stop was not caused by its software or its integration with air traffic control. The FAA also denied claims that a software update error or cyberattack triggered the temporary telecommunications outage, after Bloomberg reported the FBI was examining whether a software update was behind the disruption. Homeland Security Secretary Markwayne Mullin said there was no indication of a nefarious attack and that “this isn’t another 9/11,” though he said officials were not ruling anything out and had no timeframe for a full recovery.