The agency's notice said the ground stop was imposed "due to extraordinary circumstances and for reasons of safety." Transportation Secretary Sean Duffy said the communications problem affected all 22 regional centers that handle en route traffic, and that FAA technicians were investigating the cause.

It was the first nationwide ground stop since January 2023, when a key FAA computer system failed, and only the second since the Sept. 11, 2001, attacks. The disruption rippled through airports across North Texas and the rest of the country, stranding passengers and forcing airlines to scramble.

Homeland Security Secretary Markwayne Mullin said there was no indication so far that the outage was an attack. "It's just a telecommunications outage," he said, adding: "Everybody's safe. There's no reason to be concerned about your loved ones if they're flying."

Mullin said investigators will examine whether the failure is connected to the FAA's nationwide rollout of a new artificial intelligence system designed to identify problems and reroute planes. That system, however, operates from a single location near Washington, D.C., and only sends recommendations to the air traffic control centers, he said.

Aviation safety expert Steve Arroyo, a former United Airlines pilot, said the incident resembled a regional communications outage in September that disrupted hundreds of flights across the Northeast, but on a national scale. He said it may be tied to the aging technology controllers still rely on.

"If you don't have that ability, whether it's because the old wires that they have are just run down or for whatever reason, you've got to stop airplanes on the ground," Arroyo said, "and not put them in the air where you're going to risk not being able to communicate with them."

Unlike the Sept. 11 ground stop, Friday's order did not direct planes already in the air to land immediately, and pilots were likely still able to communicate with ground controllers at airports, Arroyo said.

The outage spotlighted the long-running effort to modernize the nation's air traffic control infrastructure. Congress approved $12.5 billion for the overhaul last year, funding the replacement of miles of old copper wiring with fiber optic lines and the gradual swap of outdated radio systems at airports nationwide. Duffy has said another $30 billion is needed to finish the work, and the White House has begun lobbying Congress to approve it.

There was no immediate estimate of how many flights were affected or how long the ground stop would last.