More than 2,000 flights were canceled and more than 10,000 delayed by 6:20 p.m. Friday after the Federal Aviation Administration (FAA) ordered a nationwide ground stop, a halt to departures, at every U.S. airport.

The action, only the second nationwide ground stop since 9/11, followed a communications issue affecting air traffic control (ATC), the system coordinating aircraft in flight.

How large was the disruption?

FlightAware recorded the cancellations and delays as of 6:20 p.m., and lawmakers demanded answers on Friday evening. Earlier that afternoon, the FAA had already stopped departures at Chicago's O'Hare and Midway airports because of an equipment outage.

Flightradar24 counted 3,100 aircraft airborne and bound for U.S. airports earlier in the disruption. No further takeoffs were taking place at the time of reporting, while flights already in the air were to be diverted to other airports.

The FAA said the stop reflected extraordinary circumstances and reasons of safety. It gave safety as the basis for the broad halt without identifying the affected component, the extent of the outage or the cause.

Transportation Secretary Sean Duffy said on X that the communications issue was affecting all Air Route Traffic Control Centers, the facilities responsible for flights above 18,000 feet. The United States has 22 such centers. Duffy said FAA technicians were investigating the problem.

What caused the nationwide stop?

Department of Homeland Security Secretary Markwayne Mullin said there was no indication yet of a nefarious attack and that everyone was safe. "This isn't another 9/11. I cannot repeat that enough," he told reporters.

As of then, the government's working explanation was a telecommunications outage, Mullin said, though the government would investigate the root cause and all possibilities. When a reporter asked whether a cyberattack could be responsible, he said investigators were not ruling it out.

Mullin said investigators had to assume the possibility during the inquiry, but nobody knew the cause. That distinction left the central question unanswered: what failed across the control network.

The previous nationwide stop came in 2023, when the FAA's Notice to Air Missions (NOTAM) system went down and pilots could not obtain pre-departure paperwork. Friday's order concerned a communications issue instead.

AirSpace Intel, the contractor behind the SMART artificial intelligence system rolling out at facilities nationwide, said its technology had not triggered the stop. Its statement said the ground stop was neither based on nor a result of the SMART system, any company product or its integration with the FAA and ATC.

Mullin said the government was examining whether the technology rollout had any connection to the disruption. The contractor's denial answered one question but left the central one open: why communications failed across the centers.

Canada's civil air navigation service, NAV Canada, said it was monitoring the FAA order and that Canadian systems were unaffected. It continued to support the safe movement of air traffic within Canadian airspace, including any aircraft that might be diverted.

Pilots already airborne had flight plans and were prepared to continue their routes in an emergency, ABC News contributor John Nance said. He said all pilots in that environment know what to do and described the process as predictable, because air traffic control expects each aircraft eventually to descend and seek a pathway.

Who is demanding answers?

Sen. Tammy Duckworth, an Illinois Democrat and ranking member of the Aviation Subcommittee, said the safety of U.S. skies is built on redundancies. She said whatever caused the stop warranted a thorough and complete investigation and that she would demand answers from the FAA.

Sen. Maria Cantwell, a Washington Democrat and ranking member of the Senate Transportation Committee, said her team was in contact with the FAA. "Technology issues related to how planes communicate through primary frequencies, backup frequencies, and other issues will be something the Commerce Committee will be reviewing," she said.

Several congressional leaders pressed for answers and accountability on Friday evening. Their demands focused on the communications failure, the scope of the stop and the FAA's responsibility for keeping the system operating.

The operational damage was clear. The cause was not. That leaves the FAA facing congressional demands for a full account of the second nationwide stop since 9/11.