The U.S. Department of Transportation fined six airlines $7.4 million and ordered them to pay a total of more than $600 million in refunds to customers who had their travel plans ruined during a chaotic stretch for air travel earlier this year.

Transportation Secretary Pete Buttigieg said in an interview Monday with "NBC Nightly News'" Lester Holt that airlines have made progress on improving the customer experience, but with Thanksgiving and holiday travel on the horizon "we’re not out of the woods yet."

Read more on this story at NBCNews.com and watch Buttigieg’s interview on “NBC Nightly News with Lester Holt” tonight at 6:30 p.m. ET / 5:30 p.m. CT.