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 lawsuit follows a $2 million DOT fine on JetBlue Airways for similar allegations.

The lawsuit and fines come at the end of the Biden administration, which has taken a harder line toward consumer protections than previous administrations.

The DOT said that Southwest’s flights from Chicago Midway…