US Department of Transportation says it plans to look into Southwest Airlines following the airline's 'unacceptable' holiday flight cancellations

• U.S. DOT tweeted Monday it will examine whether Southwest is abiding by its customer service policy.
• Southwest cancelled nearly 3,000 flights on the day after Christmas, the most of any U.S. airline.

The U.S. Department of Transportation tweeted on Monday night that it will look into whether Southwest Airlines is abiding by its customer service policy after the Dallas-based airline cancelled nearly 3,000 flights on the day after Christmas.

The DOT is "concerned by South Southwest's unacceptable rate of cancellations and delays & reports of lack of prompt customer service," the tweet read.

The tweet also linked to Southwest's customer service plan.

As of 9:45 p.m. EST on Monday night, Southwest had canceled 2,893 flights, more than any other U.S.-based airline. Delta had the second-most number of cancellations, 267.

Travelers in airports across the country complained of long lines to re-book flights and retrieve baggage and unanswered phone calls to customer service.