WASHINGTON — The U.S. Department of Transportation has questions for Southwest Airlines after mass flight cancellations that have left tens of thousands of travelers stranded for days.

The president of the union that represents Southwest’s pilots said he would welcome a congressional investigation, calling the debacle completely avoidable.

“We’ve never seen anything like this at any airline ever,” Capt. Casey Murray of the Southwest Airlines Pilots Association. “We have pilots and flight attendants sleeping in airports. … That causes fatigue.”

Murray blamed Southwest for ignoring calls to update decades-old computer software and scheduling systems.

“These meltdowns occur regularly at Southwest and we’ve been trying to offer solutions” for years,” Murray said. “This should never have happened.”

U.S. Secretary of Transportation Pete Buttigieg agreed, vowing to hold the airline accountable. He said his agency is working around the clock to help affected passengers.

“Not just making sure that anybody whose flight is canceled can get a prompt cash refund, but also making sure that your meals, your hotels, your ground transportation are covered,” Buttigieg said.

Southwest CEO Bob Jordan promised to uphold those commitments.

“We always take care of our customers, and we will lean in and go above and beyond as they would expect us to,” Jordan said.

He said the goal was to have the airline back on track before next week.

The union president said Southwest needs to take the experience as a wake-up call and be transparent about what it’s going to do to prevent a similar breakdown.

“If they don’t, they’re not going to survive,” he said.