The ground stop began around 3 p.m. and was lifted around 7:30 p.m., according to federal officials. Flight tracking data showed more than 3,000 planes were in the air in the U.S. or headed toward the U.S. when the order was issued. Life-saving and law enforcement flights were permitted to continue during the stoppage.

U.S. Secretary of Transportation Sean Duffy said it will take "some time" before travel returns to normal operations, but he expects conditions to be restored by the time Thanksgiving travel begins.

Federal officials identified the problem as outdated equipment that was overwhelmed by the volume of data it was handling, became unstable and had to be shut down. Duffy said investigators are still working to determine what was different about Friday's data load compared with any other day.

Earlier in the day, the FAA had issued a ground stop at Chicago's O'Hare and Midway airports because of equipment outages. A Milwaukee Mitchell International Airport spokesperson said the local stoppage was "due to an apparent equipment outage." Duffy posted on social media that the problem was "a communication issue impacting all Air Route Traffic Control Centers." There are 22 such centers, which monitor flights above roughly 18,000 feet; one is based in Chicago and covers Milwaukee.

It was only the second nationwide ground stop the FAA has ordered since the Sept. 11, 2001, attacks. The previous one occurred in 2023 when a system failure prevented pilots from receiving pre-departure paperwork.

Congress approved $12.5 billion last year to overhaul the aging technology that air traffic controllers rely on. Miles of old copper wiring have been replaced with fiber optic lines, and old radio systems are gradually being swapped out at airports nationwide. But Duffy said another $30 billion will be needed to finish the work, and the White House has begun lobbying Congress to approve it. On Friday evening, Duffy raised that estimate to $50 billion.

The disruption rippled through the region. The Wisconsin Badgers football team landed safely in Maryland during the ground stop, the team confirmed. FEMA activated its centers to help ensure people did not miss medications, and federal officials said they were in communication with all airlines to manage stranded passengers.

"This is one of those scenarios where this is not the fault of the airline," Duffy said.

Passengers who were on the tarmac or inside airports during the stoppage faced delays, cancellations and, for some, being stuck on planes. Travelers are advised to check flight status directly with Milwaukee Mitchell International Airport and O'Hare before heading to the airport.