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 forced airlines to cancel thousands of flights.

"The FAA Command Center nationwide ground stop has been cancelled," the agency said in an advisory. The notice said another update is expected at 9 p.m.

The ground stop took effect at 1:53 p.m., as the Kiowa County Press reported earlier Friday. The FAA 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 FAA has not said what failed.

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.

As of 6:22 p.m., 197 departures from Denver International Airport had been canceled and 341 delayed Friday, according to the flight-tracking service FlightAware. Another 165 arrivals had been canceled and 313 delayed. Nationally, FlightAware counted 5,731 cancellations of flights within, into or out of the United States.

FlightAware showed 654 United Airlines cancellations Friday, 398 for Southwest Airlines and 65 for Denver-based Frontier Airlines.

Disruptions are expected to continue into Saturday. United, 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.

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.

Homeland Security Secretary Markwayne Mullin told reporters outside the White House during the stop that there was "no indication, as of yet, that this was a nefarious act against us," CBS News reported. "It was just a telecommunication outage, as of right now," Mullin said. Microsoft said it had no indication the ground stop was related to its own recent service disruption, NBC News reported.

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.