American Airlines has issued a travel alert on Tuesday to give passengers flexibility who were impacted by a technical issue on Christmas Eve.

The airline said a “vendor technology issue” caused a temporary grounding of the airline’s flights on Tuesday morning during the busy holiday travel season.

“We have resolved a vendor technology issue that briefly affected flights this morning. We sincerely apologize to our customers for the inconvenience and have issued a travel alert to allow for additional flexibility,” American said in a post on social media.

The airline is waiving change fees for travelers booked in any fare class, including Basic Economy, who bought their ticket by Dec. 23 to travel on Dec. 24, if they:

• Book changes on Dec. 24 for travel Dec. 24 to Dec. 26
• Don’t change origin or destination city
• Don’t cancel the trip and request a refund
• Are traveling on an American Airlines flight

The cities impacted that qualify for the travel flexibility are for travel to, through or from:

• Austin, Texas (AUS)
• Boston, Massachusetts (BOS)
• Charlotte, North Carolina (CLT)
• Chicago, Illinois (ORD)
• Dallas / Fort Worth, Texas (DFW)
• Los Angeles, California (LAX)
• Miami, Florida (MIA)
• New York Kennedy, New York (JFK)
• New York LaGuardia, New York (LGA)
• Philadelphia, Pennsylvania (PHL)
• Phoenix, Arizona (PHX)
• Washington Reagan, Washington D.C. (DCA)

American Airlines said it will grant one-time change allowance for flights that qualify.

American flights were cleared to fly by federal regulators about one hour after a national ground stop order was issued by the Federal Aviation Administration.

There were 1,447 delays for flights entering or leaving the U.S. early in the day, with 28 cancellations. Snow was falling early in New York and Dallas-Fort Worth International, which is American Airlines’ main hub, was getting hit with rain.

Dallas-Fort Worth had the most delays, followed by Charlotte, North Carolina, Washington, New York, Chicago and Miami.

This will be the first holiday season since a Transportation Department rule took effect that requires airlines to give customers an automatic cash refund for a canceled or significantly delayed flight. Most air travelers were already eligible for refunds, but they often had to request them.

Passengers still can ask to get rebooked, which is often a better option than a refund during peak travel periods. That’s because finding a last-minute flight on another airline yourself tends to be very expensive.

Just before 7 a.m. Eastern time, the Federal Aviation Administration ordered all American Airlines flights grounded in the U.S. at the airline’s request. American had reported a technical issue affecting its entire system with millions traveling for the holiday.

American said in an email that the problem Tuesday morning was caused by a vendor technology issue that “impacted systems needed to release flights.”

The groundings couldn’t come at a worse time for the millions of travelers expected to fly over the next 10 days. The Transportation Security Administration expects to screen 40 million passengers over the holidays and through January 2.

Airlines expect to have their busiest days on Friday and Sunday, and on Dec. 26, Dec. 27 and Dec. 29.

Many flights during the holidays are sold out, which makes cancellations 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.

About 90% of Americans traveling far from home over the holidays will be in cars, according to AAA.

“Airline travel is just really high right now, but most people do drive to their destinations, and that is true for every holiday,” AAA spokesperson Aixa Diaz said.