Flights already in the air can land, but any planes still on the ground cannot take off. In a post on X, Transportation Secretary Sean Duffy blamed a communication issue impacting all Air Route Traffic Control Centers.

“FAA technicians are investigating the issue,” he wrote.

Despite recent hacking incidents of freezers in Europe and U.S. military bases, Homeland Security Secretary Markwayne Mullin told reporters the Trump administration doesn’t suspect foul play was involved.

He added that the issue will take some time to fix but that there is no threat of a terrorist attack.

“This isn’t another 9/11,” he said. “This is a telecommunication outage.”

Still, the massive disruption comes as airlines in Europe suffered a wave of phony bomb threats this week. While they all appeared to hoaxes, 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.”

Meanwhile, consumers are still grappling with the aftermath of a Microsoft outage that shut down email, collaboration and AI functions. The company said Friday it had no indication that the ground stop is related to its global outage.

The National Weather Service also said there has been no disruption to its radar systems Friday, denying talk online that it had caused the FAA communications issue.

But speculation swirled elsewhere. Just days before the ground stop, the FAA announced it would deploy an AI tool to help air-traffic controllers, though it’s not clear if that was a factor on Friday.

The FAA last issued a nationwide ground stop in 2023, when files related to an alert system for pilots was unintentionally deleted. It delayed thousands of flights, but they resumed soon after the disruption.

Earlier this year, flights at New York airports were halted when a contractor accidentally cut a fiber communications line in New Jersey.