The US Department of Transportation (USDOT) is suing Southwest Airlines for “illegally operating multiple chronically delayed flights and disrupting passengers’ travel,” according to a press release.

The Department of Transportation on Wednesday sued Southwest Airlines, alleging the carrier operated chronically delayed flights, and fined Frontier Airlines for late-arriving flights.

The Transportation Department is stepping up enforcement of persistent flight delays with a civil lawsuit against Southwest Airlines and a fine against Frontier Airlines.

A lawsuit filed against Southwest alleges the airline operated multiple “chronically delayed” flights affecting thousands of passengers.

The Department of Transportation sued Southwest Airlines Wednesday alleging it deceived customers by offering flights it knew it could not make on time.

Co. was sued by the US Transportation Department for allegedly violating rules that require airlines to set and meet realistic flight schedules. The carrier was responsible for “unlawful chronic flight delays” that disrupted passengers’ travel plans,

The Transportation Department said in a lawsuit that two of the airline’s routes were consistently delayed in violation of federal rules.