The Federal Aviation Administration halted all departures at U.S. airports Friday afternoon, citing a communication problem affecting its air traffic control system.

Transportation Secretary Sean Duffy said the issue affected “all Air Route Traffic Control Centers” and that FAA technicians were investigating. The agency’s advisory cited “extraordinary circumstances and for reasons of safety.” It said departing traffic would stay grounded until further notice and that inbound international flights would be handled case by case.

The stoppage began shortly before 4 p.m. FlightRadar24 showed more than 1,000 aircraft airborne and headed for U.S. airports. Flighty counted more than 30,000 departures and arrivals scheduled for the rest of the day. Some airborne flights were still landing, including at O’Hare. In Denver, passengers were taken off planes, and some began rebooking for Saturday.

Earlier Friday, the FAA had halted flights to Chicago’s O’Hare and Midway airports because of equipment problems. It had also reported equipment-related delays at Newark, Las Vegas, Teterboro, Orlando, Houston and Phoenix.

It is only the third nationwide ground stop in recent history. The others came on Sept. 11, 2001, and Jan. 11, 2023, when a system that alerts pilots to safety issues failed.