The U.S. Department of Transportation filed a lawsuit against Southwest Airlines Wednesday alleging the company engaged in deceptive practices by offering flights it knew it could not make on time, according to federal court documents filed in the Northern District of California.

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.

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.