The FAA issued the ground stop for all departing flights, citing "extraordinary circumstances and for reasons of safety." The order was posted to the agency's National Airspace System Status website and triggered cascading delays at major airports from Boston to Los Angeles.

Transportation Secretary Sean Duffy said the airspace reopened at 8:15 p.m. Eastern time. "Now does that mean every flight is going to fly right now? It's going to be slow," Duffy said at a news briefing. "So if you're watching, you're at an airport, check with your airline."

Duffy attributed the disruption to outdated air traffic control technology and routers overwhelmed by data. "What we have is an incredibly old air traffic control system," he said. "Our routers actually got overloaded with data and … they started to give us inconsistency in the data from those routers. And because of that, we shut the system down."

An update to the private communications network connecting the air traffic control system caused it to reboot, leading to a crash and communications outage, according to multiple sources. The system is operated by L3Harris, which did not immediately respond to a request for comment.

Homeland Security Secretary Markwayne Mullin told reporters outside the White House that there was no indication of foul play. "There was no indication, as of yet, that this was a nefarious act against us. It was just a telecommunication outage, as of right now," Mullin said. "There is no indication, we had no reason to believe, that as of right now, that this was a threat to the United States."

Mullin said all airborne flights were being handled on a case-by-case basis and would be able to land safely. "We are assured that everybody is safe," he said. "Everybody that's going to be in the air, that's currently on the plane, the loved ones, as I said, we're going to get you on the ground and we're doing it in a safe manner."

Flight tracking site FlightRadar24 reported about 3,100 aircraft were airborne and bound for U.S. airports when the ground stop began. Within a short time Friday, more than 10,000 delays and over 4,000 cancellations were listed, according to FlightAware.

International flights departing from the East Coast were permitted to fly over the Atlantic Ocean, while flights from Jacksonville, Houston, Miami and San Juan could depart southbound over the Gulf and Atlantic, a source said.

Robert Sumwalt, a former chair of the National Transportation Safety Board, called the event unprecedented. "I was an airline pilot for 24 years. I was in the air on 9/11. That was the last major ground stop," Sumwalt said. "This is absolutely unbelievable that this could happen in this day and age."

Duffy renewed his call for Congress to approve emergency funding to modernize the nation's air traffic control system. "But you will have more of these," he warned. "You will have more of them unless the American people and the Congress tell us to build."

Duffy said the FAA would move as many flights as possible Friday night depending on crew capacity, and that airlines would begin repositioning aircraft Saturday morning. He expected operations to be "back up and operational as we come into the heat of travel" during the Thanksgiving holiday.

The disruption recalled a January 2023 nationwide ground stop that lasted a few hours after the FAA's flight personnel alert system went down overnight. Friday's event, however, was far more extensive, with the entire national airspace affected during the busy pre-holiday travel period.

Shortly before the nationwide order, both O'Hare International and Midway International airports in Chicago were issued ground stops because of an equipment outage.

The FAA's Command Center alert stated: "By order of the Federal Aviation Command Center all airports are ground stopped. All departing traffic should remain ground stopped until further notice. Inbound international flights will be managed on a case by case basis."

For travelers, the agency and airlines urged passengers to check flight status before heading to airports, as the system worked to unwind the day's disruptions.