The Federal Aviation Administration lifted a nationwide ground stop Friday after more than four hours, ending a rare halt that disrupted air travel across the country. The agency had stopped departures following communication problems affecting multiple Air Route Traffic Control Centers.

Transportation Secretary Sean Duffy said aging air traffic control equipment became overloaded with data, creating instability in radar information and communications with aircraft. The FAA halted nationwide departures out of what officials repeatedly described as an abundance of caution.

The FAA initially said the order was issued because of “extraordinary circumstances and for reasons of safety” involving “communication issues.” The agency did not initially say how long the halt would continue, directing that planes should not depart “until further notice.”

Duffy said the problems began at 3:36 p.m. ET, when air traffic control facilities nationwide experienced “instability for our radar” and communications. He blamed aging analog telecommunications equipment, including switches and routers that became overloaded.

“It’s the old hardware that went down,” Duffy said. He added that newer equipment installed under the Trump administration was not affected and renewed his call for Congress to provide more money to modernize legacy equipment.

Earlier in the week, Duffy told Politico that the administration was lobbying Congress for $30 billion to upgrade air traffic control. “You will have more of them unless the American people and the Congress tell us to build, and I can’t wait for that,” he said of potential future ground stops.

More than 3,500 aircraft were already in the air and heading toward U.S. airports when the ground stop was issued, according to Duffy. He said the FAA never lost all radar or telecommunications capabilities and continued allowing critical flights.

Those flights included medical evacuations and shipments of lifesaving medications. International flights arriving in the United States would “be managed on a case-by-case basis,” the FAA said when it issued its initial alert.

A Justice Department source told MS NOW that officials were not tracking any potential connection to terrorism and described the problem as a “frequency issue.” Homeland Security Secretary Markwayne Mullin said officials had found “zero evidence” of a cyberattack or rogue artificial intelligence, although he did not definitively rule out either possibility.

Mullin insisted that passengers were not in danger and said the FAA acted cautiously. He also said the Federal Emergency Management Agency had activated its centers to assist stranded travelers, including passengers who might need medication.

FEMA was coordinating with airports, cities and states to address travelers’ needs, according to Mullin. Meanwhile, Duffy warned that restoring regular flight operations “is going to be slow,” with airlines prioritizing larger flights based on crew availability.

“It’s going to take us some time to unwind what’s happened this afternoon,” Duffy said. He expected operations to improve Saturday and return to normal before Thanksgiving travel.

Two people at the FAA confirmed that the interruption was only the second nationwide ground stop since the Sept. 11, 2001, terrorist attacks. The previous nationwide ground stop occurred in 2023.

President Donald Trump had not publicly addressed the disruption as of shortly before 9 p.m. ET. Duffy said Trump had been in frequent contact with him and Mullin throughout the disruption and had communicated with both officials multiple times.

Democratic lawmakers demanded answers about the failure. Sen. Cory Booker of New Jersey called the situation a “significant breach or breakdown” and said, “Whether this is a breach or a breakdown, either one is unacceptable.”

Sen. Maria Cantwell, the top Democrat on the Senate Commerce, Science and Transportation Committee, said the committee would review technology issues involving primary frequencies, backup frequencies and other communication concerns. Rep. Jim Himes, the ranking Democrat on the House Intelligence Committee, said he had not been told what caused the stoppage.

Airports advised travelers that departing flights were being held and urged them to contact their airlines. American Airlines and Delta said aircraft already in the air would continue to their destinations, while JetBlue canceled its remaining Friday flights except those heading to Europe.

Travelers across the country reported lengthy delays and sparse information. Passengers were ordered off one flight at Los Angeles International Airport, while a traveler at Jacksonville International Airport was told their departure would be delayed by at least seven hours.

FlightAware reported more than 9,500 delays and more than 5,300 cancellations as of about 9:25 p.m. ET. The staggering disruption lingered even after the FAA lifted the order, leaving airlines to slowly untangle the nationwide backlog.