The company has not said why it was stopping all flights. Numerous passengers were posting on social media that their flights had been stuck on the runway at various airports and were now being sent back to the gate.

“An estimated timeframe has not been provided, but they’re trying to fix it in the shortest possible time,” the company said in a post on X, responding to a question from a stranded flyer.

Shares of the carrier were down 3.8% before the bell. A notice on the U.S. Federal Aviation Administration website merely said the company had requested a nationwide groundstop, without giving a reason.

As of 7:30 a.m. ET, American had not made a formal statement on social media, and was only responding to comments on X as numerous users posted there, as well as on Bluesky and Facebook.

“Hey, @AmericanAir just tell us whether we should go home or not. Please don’t make us wait in the airport for hours,” wrote one user.

American operates thousands of flights per day to more than 350 destinations in more than 60 countries.

The grounding comes months after airlines were hit by a global tech outage tied to Microsoft’s Azure cloud platform and a software issue at cybersecurity firm CrowdStrike.

The U.S. FAA was not immediately available for further comment.

Two years ago, Southwest Airlines experienced a meltdown with its systems during the holidays that led to 16,900 flight cancellations and stranded 2 million passengers. It was eventually fined $140 million in the largest-ever civil penalty for a travel disruption.