Takeoffs from airports throughout the US largely stopped on Friday after the FAA issued a national ground stop “due to extraordinary circumstances and for reasons of safety,” while planes in the air will be allowed to land. The FAA tweeted just after 5PM ET that, “There has been a temporary telecommunications outage – out of an abundance of caution all flights have been halted. Every flight has been accounted for and international flights will be handled on a case by case basis.” The flight tracking site FlightAware lists 10,494 US flights as delayed and 3,396 flights canceled as of 6:16PM ET.

According to CNN, the last nationwide ground stop in 2023 lasted for about 90 minutes after the NOTAM (Notice to Air Missions) system for pilots went down, and the last one prior to that was after the attacks on September 11th, 2001.

Speaking to reporters at the White House on Friday afternoon, Homeland Security Secretary Markwayne Mullin told reporters that there is currently no reason to suspect a cyberattack or other national security threat, or any connection to the recent Microsoft Outlook outage, but that so far no cause has been ruled out for the “telecommunications outage.”

When asked about any possible connection to the cloud-based AI Strategic Management of Airspace, Routes and Trajectories (SMART) system that was activated nationwide on November 10th, he said they were looking into that. Air Space Intelligence (ASI), the company that designed the system and was awarded a 12-year $875 million contract in June to deploy it, denied any connection, tweeting “The current ground stop is not based on or a result of the SMART system, any product of ASI or our integration with FAA and ATC.”

Mullin also said international flights and departures are being handled on a case-by-case basis, while NBC News and the New York Times report that some flights headed to Europe and Asia have taken off from Boston and New York.

Jon Ostrower of The Air Current reports that the outage began with the Chicago air traffic control center, followed by centers in Boston, Washington, New York, and Minneapolis, and as the FAA issued a ground stop for aircraft flying east, more facilities began reporting problems. He says the problems are disrupting both the radar and radio frequencies used by air traffic controllers and pilots, while a CNN update on Friday cited an unnamed source claiming the FAA “doesn’t know yet what is going on,” with no clear answers about the cause.

Updated, Friday, November 13th: Added additional details.