American Airlines grounded all flights across the country Tuesday morning due to a technical issue, impacting millions on one of the busiest travel days of the year.

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

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

The company has not yet provided any estimated timeframe for a resolution.

This is a developing story. Refresh for updates.