The Federal Aviation Administration halted all departures at U.S. airports on Friday after a communications failure struck every Air Route Traffic Control Center in the country, leaving millions of travelers in limbo.

The order, a nationwide ground stop, allows flights already airborne to land but blocks every aircraft still on the ground from taking off. Transportation Secretary Sean Duffy announced the cause on X: a communication issue impacting all Air Route Traffic Control Centers at once.

"FAA technicians are investigating the issue," Duffy wrote.

Is foul play suspected?

No, according to the Homeland Security Secretary. Markwayne Mullin told reporters the Trump administration does not suspect foul play, despite recent hacking incidents targeting freezers in Europe and U.S. military bases. He cautioned the fix will take time.

"This isn't another 9/11," Mullin said. "This is a telecommunication outage."

He added there is no threat of a terrorist attack. That assurance matters because Friday's disruption landed amid an already tense week for aviation security on the other side of the Atlantic.

What is happening in Europe?

European airlines have absorbed a wave of phony bomb threats this week. All of them appeared to be hoaxes, but at least one flight was diverted to a military base in France. Spanish Foreign Minister Jose Manuel Albares said Friday that Europe appeared to be facing "hybrid aggression and a form of psychological warfare."

Could the Microsoft outage be responsible?

Microsoft said Friday it had no indication the ground stop is related to its global outage, which shut down email, collaboration and AI functions and continues to affect consumers. The National Weather Service also denied online talk that its radar systems caused the FAA problem, saying it suffered no disruption Friday.

Speculation has swirled regardless. Just days before the ground stop, the FAA announced it would deploy an AI tool to help air-traffic controllers. It is not clear whether that tool was a factor in Friday's failure.

How rare is a nationwide ground stop?

The FAA last issued one in 2023, when files related to a pilot alert system were unintentionally deleted. That incident delayed thousands of flights, which resumed soon after the disruption ended.

There is a closer precedent still. Earlier this year, flights at New York airports were halted when a contractor accidentally cut a fiber communications line in New Jersey. Friday's event is far broader in scope, touching every airport in the national system rather than a single region.

What happens next?

For now, the situation is static: planes land, nothing takes off, and technicians work the fault. Airlines and airport operators must absorb the backlog once the order lifts, a process that took hold quickly after the 2023 incident.

Friday's ground stop is a telecommunication outage, in the administration's words — but it freezes the world's largest aviation market.