A “technical issue” led American Airlines to briefly issue a nationwide ground stop on Christmas Eve morning, stopping more than 100 flights on one of the busiest travel days of the year.

But what exactly happened, how long did the ground stop last, and have flights resumed? Here’s what we know.

When was the American Airlines ground stop issued?

The ground stop was issued by the Federal Aviation Administration just before 6:30 a.m. CT Tuesday.

“A technical issue is affecting American flights this morning,” the airlines said. “Our teams are working to resolve the issue as quickly as possible, and we apologize to our customers for the inconvenience.”

An advisory issued by the Federal Aviation Administration showed the company issued a request for a full ground stop of its flights.

In social media replies to frustrated travelers, the airline said: “Our team is currently working to rectify this. Your continued patience is appreciated.”

The ground stop, according to the time stamps on the FAA’s orders, lasted exactly one hour.

According to NBC News, American flights had begun to board shortly approximately an hour later, with the airline telling NBC News it was seeing improvement.

Why did American Airlines issue a ground stop?

The exact nature of the technical problem and what systems it may have impacted was not immediately disclosed.

In a report from NBC News, social media users delayed at their gates had reported issues with having their boarding passes scanned. 

Social media users reported being instructed to deplane at airports across the country, including Chicago O’Hare International Airport.

In December 2022, Southwest Airlines stranded 2 million travelers, and Delta Air Lines suffered a smaller but significant meltdown after a worldwide technology outage in July caused by a faulty software update from cybersecurity company CrowdStrike.

Many flights during the holidays are sold out, which makes cancellations even more disruptive than during slower periods. Even with just a brief outage, the cancellations have a cascading effect that can take days to clear up.

Delays, cancellations at Chicago airports

The grounding occurred with millions of travelers expected to fly over the next 10 days.

As of 7:30 a.m., CT Tuesday, O’Hare reported one cancelation and average delays of 18 minutes, the website flychicago.com said. No cancelations were reported at Midway Airport, the site showed.

The website Flight Aware, which tracks flights across the country and globe, showed 24 cancellations at 7:30 a.m. Tuesday within, into, or out of the U.S.

Earlier Tuesday, that number was more than 100.

More than 3 million passengers are expected to travel through Chicago’s O’Hare and Midway International Airports through Jan. 2, the Chicago Department of Aviation said. The CDA projects Sunday, Dec. 29 to be the busiest day at Midway, when more than 51,000 people will pass through the airport.

Nationally, the Transportation Security Administration expects to screen 40 million passengers over the holidays and through January 2.