Takeoffs from airports throughout the US paused on Friday after the FAA issued a national ground stop “due to extraordinary circumstances and for reasons of safety,” while planes in the air will be allowed to land. US Transportation secretary Sean Duffy tweeted:

The FAA issued a nationwide ground stop for all U.S. airports following a communication issue impacting all Air Route Traffic Control Centers. FAA technicians are investigating the issue. We will provide immediate updates as we have them.

Jon Ostrower of The Air Current reports that the FAA has said to airlines that the stop is “due to several impacts to ATC Frequencies and/or Radar capabilities.” The flight tracking site FlightAware lists 7,884 US flights as delayed as of 4:56PM ET. Speaking at the White House, Homeland Security Secretary Markwayne Mullin told reporters that there is currently no reason to suspect a cyberattack or other national security threat, but that so far no cause has been ruled out for the outage.