The Federal Aviation Administration ordered the ground stop at about 3:36 p.m. Eastern time after communications among air traffic control facilities became unstable. The restriction was lifted at 8:19 p.m. Eastern, according to an FAA advisory.

"We didn't lose all telecom. We didn't lose all radar. But that instability, because of an abundance of caution and making sure we keep the American flier safe, we did the ground stop," Transportation Secretary Sean Duffy said at a White House news conference Friday evening.

Duffy described the cause as aging analog telecommunications equipment whose routers became overwhelmed with data and began producing inconsistent information. "It's the old hardware," he said.

The FAA initially posted an advisory declaring that "due to extraordinary circumstances and for reasons of safety," all airports were ground stopped and all departing traffic should remain grounded until further notice. Inbound international flights were to be handled case by case.

Homeland Security Secretary Markwayne Mullin told reporters there was no indication the outage was caused by a cyberattack, though officials were not ruling anything out. "There's no chatter, we haven't seen any footprints," Mullin said. "It appears that this was mechanically driven. However, we're still looking."

Mullin sought to calm public anxiety, saying the stoppage did not stem from a threat to Americans' safety. "We were wanting to assure all the American people this isn't a safety threat to the American people or another 9/11," he said. "I think that's what a lot of people are trying to make the comparison to, and that's not the case."

Flights already in the air continued to land during the stoppage, and officials said some international departures were cleared on a case-by-case basis. Europe suspended all U.S.-bound flights during the ground stop.

Airlines moved quickly to issue travel waivers allowing passengers to change plans without fees. American Airlines said most departures remained paused and that "unavoidable cancellations" were expected. United said new domestic flights would not depart while the stop remained in effect and that diversions were occurring because of congestion at its hubs. Delta also issued waivers.

The cascading disruption hit ground transportation as well: Amtrak's website and app were inundated with stranded travelers trying to book tickets, according to the airline advisories and officials.

Duffy predicted the system would be back to normal in time for the busiest travel period of the year, but acknowledged a significant cleanup. "I believe that it's going to take us some time to get back to capacity and then get people who were disrupted over the course of the last eight hours, to get them back to their destination," he said. "I think we're going to be back up and operational as we come in to the heat of travel during Thanksgiving."

The ground stop was the first nationwide halt since January 2023, when a key FAA computer system malfunctioned. Before that, the last such order came after the Sept. 11, 2001, attacks.

A ground stop differs from a ground delay in that it completely halts departures rather than spacing them out. The FAA says ground stops are typically used to control traffic volume, buy time for a longer-term fix, or when an airport's acceptance rate drops to zero.

For local travelers, the practical advice from airlines and officials was to check flight status before heading to the airport, take advantage of waivers to rebook, and expect crowded terminals and full planes as the system absorbs the backlog in the days ahead.