Pete Buttigieg among thousands who saw their flights canceled this weekend

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At least 14,000 domestic flights were canceled or delayed in the United States this holiday weekend, leaving thousands of passengers stranded in part because of staffing issues for airlines in the air and on the ground.

More than 900 flights were canceled on Sunday alone. Delta was among the airlines with the most cancellations, with at least 200 flights grounded.

"A variety of factors continue to impact our operations, including challenges with air traffic control, weather and unscheduled absences in some work groups. Canceling a flight is always our last resort, and we sincerely apologize to our customers for the inconvenience to their travel plans," a Delta airline spokesperson said in a statement.

Since the start of June, U.S. airports have seen an average of 2.2 million passengers every day.

Amid the cancellations, Department of Transportation Secretary Pete Buttigieg met with airline executives on Friday.

Buttigieg is advising airlines to meet their flight schedules and hire more customer-service workers. He said that if they do not meet these requirements by the Fourth of July weekend, the department could enforce fines on the airlines.

The day after he met with airline leaders, his own flight was canceled. Buttigieg was forced to drive from Washington D.C. to New York.

CBS News senior travel adviser Peter Greenberg said that airlines need to hire more in all departments to avoid more cancellations.

"You can hire as many customer service agents. You want to say, 'I'm sorry' but if you don't fix the root problems, the apologies will continue," he told CBS News' Elise Preston. "You gotta be able to find pilots. There's a shortage. You have to be able to find people who work under the wing — ground handlers, baggage loaders."

Travel experts recommend travelers book the first flight out to avoid cancellations. Due to the increase in summer travel, the Transportation Security Administration recommends that travelers arrive at least three hours before departure.

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