Following a brief outage that grounded all American airlines flights during one of the busiest travel periods of the year, American Airlines says it has resumed service Tuesday morning. American Airlines said a “vendor technology issue” was responsible for delaying all of its flights in the United States.

Bad weather made delays worse after problems with systems used to release flights on one of the busiest travel days of the year.

The Federal Aviation Administration has ordered all American Airlines flights grounded in the U.S. after the carrier reported a technical issue affecting its entire system with millions traveling for the holiday.