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

The agency said the U.S. District Court lawsuit it filed in California on Wednesday alleges that Southwest illegally operated chronically delayed flights and disrupted passengers' travel plans. It says it's seeking "maximum civil penalties."

"Airlines have a legal obligation to ensure that their flight schedules provide travelers with realistic departure and arrival times," Transportation Secretary Pete Buttigieg said in a statement. "Today's action sends a message to all airlines that the department is prepared to go to court in order to enforce passenger protections."

The Transportation Department said its investigation found that Dallas-based Southwest operated two chronically delayed flights - one between Chicago Midway International Airport and Oakland, California, and another between Baltimore, Maryland and Cleveland, Ohio.

Both flights were chronically delayed for five straight months and together resulted in 180 flight disruptions for passengers between April and August 2022, the agency said.

In a statement, Southwest said it was disappointed that the agency focused on flights from more than two years ago and pointed instead to its long-term record.

"Since DOT issued its Chronically Delayed Flight (CDF) policy in 2009, Southwest has operated more than 20 million flights with no other CDF violations. Any claim that these two flights represent an unrealistic schedule is simply not credible when compared with our performance over the past 15 years," the company said. "In 2024, Southwest led the industry by completing more than 99% of its flights without cancellation."

The department fined Frontier Airlines $650,000 in civil penalties, with $325,000 to be paid to the U.S. Treasury and the remaining $325,000 to be suspended if the carrier does not operate any chronically delayed flights in the next three years. Frontier, based in Denver, declined to comment.

The announcement comes less than two weeks after the Transportation Department fined JetBlue $2 million for chronic delays, the first time it imposed a penalty for lateness on specific routes. The agency cited JetBlue's chronic delays on "unrealistic scheduling." JetBlue then said the government, which operates the air traffic control system, shares the blame for late flights.

Aviation-data provider Cirium said in a report released this month that Southwest ranked fifth among the 10 North American airlines it evaluated for their on-time performance, with 77.8% of arrivals and a little under 77% of departures last year taking place in a timely way. By comparison, top-ranked Delta Air Lines scored 83.5% for arrivals and 83.7% for departures.

Last year, federal regulators increased their scrutiny of Southwest Airlines after a series of incidents, including flying at very low altitudes while still miles away from an airport. The Federal Aviation Administration said in July it wanted to ensure the carrier was complying with federal safety regulations. The FAA declined to provide details, but noted that it continually adjusts oversight of airlines based on risk.