American Airlines Group Inc. said a technical vendor glitch was the cause of a brief groundstop on all U.S. flights on Christmas Eve, one of the busiest days of the holiday travel season.

The carrier said Tuesday the delay lasted about one hour, but the issue has been resolved and flights have resumed. It blamed an unspecified technology issue impacting its ability to release flights.

“We sincerely apologize to our customers for the inconvenience this morning,” the company said in a statement. “It’s all hands on deck as our team is working diligently to get customers where they need to go as quickly as possible.”

The airline didn’t say if it expected any delays or other knock-on effects. American Air had 3,320 domestic flights and 581 international flights scheduled for the day, according to aviation data provider Cirium.

American Airlines shares fell 2.1% to $16.88 as of 9:37 a.m. in New York.

American Air had said earlier it was experiencing technical issues affecting all of its flights, disrupting operations on a day where the Federal Aviation Administration expects to see about 30,000 U.S. flights across all carriers.

Some online postings said the airline was suffering a software outage preventing it from calculating weight and balance requirements for its flights.

The incident comes after Delta Air Lines Inc. suffered a multi-day grounding in July affecting thousands of travelers due to a software glitch. And it is reminiscent of a year-end meltdown at Southwest Airlines Co. two years ago involving computer system issues.

(With assistance from Jason Kao.)

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