The Federal Aviation Administration issued a nationwide ground stop for all flights in the United States on Friday due to communication problems.

In a statement, the FAA said all flights were grounded “due to extraordinary circumstances and for reasons of safety.” 

Transportation Secretary Sean Duffy said in an X post that a communications problem was affecting Air Route Traffic Control Centers. 

The stop is the most severe shutdown of air travel since the Sept. 11 terrorist attacks in 2001, according to reporting from The New York Times.

“We have no reason to believe as of right now that this was a threat to the United States,” Homeland Security Secretary Markwayne Mullin said at the White House on Friday afternoon. “This was a telecommunication outage throughout FAA. Now, out of abundance of caution, we have grounded all the flights, and all the flights are accounted for.” 

Mullin also said there was no indication the issue was connected to the widespread Microsoft outage earlier this week. 

U.S. Sen. Tammy Duckworth, an Illinois Democrat who serves as ranking member on the Aviation Subcommittee, said that the ground stop “will warrant a thorough and complete investigation.”

While the cause of the issue is not yet known, some have noted that the FAA recently expanded the use of an artificial intelligence tool to assist in managing air traffic. The agency has said the tool only suggests routes and will not affect safety, but some Democrats have raised concerns about its rollout, Gizmodo reported. 

When asked about the possibility that the issue could be related to AI, Mullin said “you just can’t rule out anything at this point.”

“This is unprecedented when you see something like this,” Mullin said. “It’s going to take some time, and so this isn’t going to just be resolved and turned around within the hour.”