The Federal Aviation Administration lifted its nationwide ground stop at 6:16 p.m. Mountain time Friday, ending a halt on departures from every U.S. airport that lasted more than four hours, and Transportation Secretary Sean Duffy blamed overloaded, aging communications equipment for the failure.

"We use 1980s little copper lines. It's an old system that went down and went down everywhere. It got overloaded," Duffy told reporters at the White House, ABC News reported. "Our switches, our routers, actually got overloaded with data," he said, according to NBC News.

Duffy said "all of the new equipment that we put in, none of that went down," and called for emergency funding from Congress to replace the system. "This is an emergency," he said. "I want $50 billion." Congress approved $12.5 billion for air traffic control upgrades last year.

"The FAA Command Center nationwide ground stop has been cancelled," the agency said in an advisory. Recovery will be gradual. "I believe that it's going to take us some time to get back to capacity," Duffy said, adding that "hopefully tomorrow we are going to start to see far better movement."

As of 7:39 p.m., 215 departures from Denver International Airport had been canceled and 336 delayed Friday, according to the flight-tracking service FlightAware. Another 183 arrivals had been canceled and 308 delayed. Nationally, FlightAware counted 6,755 cancellations of flights within, into or out of the United States.

Homeland Security Secretary Markwayne Mullin said investigators had not found signs of an attack but had not ruled one out. "It appears that this was mechanically driven. However, we're still looking, so we're not going to rule that out," Mullin said, ABC News reported. Microsoft said it had no indication the ground stop was related to its own recent service disruption, and the company behind the FAA's new SMART air traffic management tool said the ground stop "is not based on or a result of the SMART system," NBC News reported.

The ground stop took effect at 1:53 p.m., as the Kiowa County Press reported earlier Friday. The FAA had blamed a "temporary telecommunications outage" affecting the Air Route Traffic Control Centers, which manage high-altitude traffic between airports. Colorado is handled by the Denver center in Longmont.

The first sign of trouble in FAA records came in Chicago. At 1:17 p.m., about 35 minutes before the nationwide order, the FAA halted flights headed to Chicago Midway International Airport from across the country, citing an equipment outage, according to an FAA advisory.

Shortly before the nationwide order, a notice from the National Weather Service hinted at the trouble. At 1:44 p.m., the Weather Service office that serves the Boston area reported that data from the FAA's weather radar at Boston Logan International Airport had been restored, and that "Boston Center reported teleco issues actively being worked." Boston Center is the FAA facility that handles high-altitude traffic over New England, and "teleco" is shorthand for telecommunications. The notice warned that further interruptions were possible.

Disruptions are expected to continue into Saturday. United Airlines, which operates a hub at Denver, said it planned to cancel most of its remaining Friday flights, with exceptions for some international flights, ABC News reported. JetBlue said it was canceling all remaining Friday flights except Europe-bound departures from New York and Boston.

"We expect disruptions to continue into tomorrow, including additional cancellations," JetBlue said in a statement.

After the nationwide stop ended, the FAA briefly held United flights bound for Chicago O'Hare International Airport and, at the airline's request, Alaska Airlines flights bound for Seattle and San Diego, according to FAA advisories.

Some airlines are waiving change fees. United told passengers they can reschedule without change fees or fare differences on a United flight departing between November 12 and November 20, in the same cabin and between the same cities, and Delta is allowing free changes for Friday and Saturday travel, ABC News reported. Travelers should check with their airline before leaving for the airport. The FAA posts current airport conditions at nasstatus.faa.gov.

While the stop was in effect, flights already in the air were allowed to land, and a limited number of flights over the ocean were cleared to leave, including Pacific-bound flights from San Francisco, flights to and from Hawaii and some transatlantic flights from Boston, according to ABC News.

Nationwide ground stops are rare. The last one came in January 2023, when the failure of the FAA's Notice to Air Missions system held departures across the country for about 90 minutes. Before that, the FAA cleared the skies after the September 11, 2001, terrorist attacks.