Holiday travel nightmare continues for many at Denver International Airport, Department of Transportation looking into Southwest's rate of cancellations

For many Denver International Airport travelers, it feels like a bad Christmas gift that keeps on giving.

"We flew out of Baltimore and we got delayed, and we got here and it was all great until we hit town here," said Aggie Bothwell, who is visiting from Virginia. "Just none of the turnstalls are going and there is luggage all the way to the wall, and tons of people."

After days of cancellations and delays at the airport, much of that anger is now pointing to Southwest Airlines, as data from FlightAware shows the airline cancelled more than 50% of its flights with traffic through the airport.

On Monday, the United States Department of Transportation announced that it's looking into the airline after what they call an "unacceptable rate of cancellations and delays & reports of lack of prompt customer service."

The organization tweeted, "The Department will examine whether cancellations were controllable and if Southwest is complying with its customer service plan."

Southwest customers like Susan and Rick Biegler said the last few days have been a nightmare. "We walked in at 5 this morning and it was crazy," said Susan, who is traveling with her husband. The couple, who live in Las Vegas, have been waiting for a flight since Saturday. Rick had to be rushed to the hospital for care, after dealing with days of cancellations.

Patricia Mammenga was also stranded at the airport. She said she had to sleep by baggage claim because the company no longer had any vouchers for hotels.

"People were on the phone to call for like 4 hours, and still did not get an answer," Mammenga said.

On Monday, near baggage claim, Southwest employees were also showing their frustrations. Lyn Montgomery, the president of the TWU Local 556, Southwest Airlines flight attendants union, released a statement criticizing the company and its leadership.

"It's deplorable. These are horrible conditions for anyone to be asked to travel in and anybody to be able to work in," Montgomery said.

While an airline spokesperson said the operational failures are due to "extreme winter weather," Montgomery said the problems showcased over the holiday weekend and on Monday stem from outdated systems that have been an ongoing problem for years, since 2016. Also, a lack of adequate staffing and miscommunication with staff.

"They have taken on more than they can chew, because we are still looking at expanding. We've hired 3,900 flight attendants this year and are continuing to hire more but when weather hits, we can't handle the operation we have," said Montgomery. "So frankly many of us are very concerned about the expansion when we have so many disastrous issues with the operations we currently have."

In a statement, an airlines spokesperson said they "were fully staffed and prepared for the approaching holiday weekend," and that "we'll work to make things right for those we've let down, including our employees."

The union and Southwest customers are calling for action, instead of what they say is the company's usual lip service.

"There's no answer, they are in big trouble and I hope they get it straightened out," Susan said.