Southwest Airlines was sued in federal court by the Department of Transportation, while Frontier was hit with a $650,000 fine.
The DOT's moves come weeks after issuing a multi-million dollar fine against JetBlue.
United States and U.S. Department of Transportation DOT announced a lawsuit against Southwest Airlines (LUV) for illegally operating multiple
The agency is seeking civil penalties against Southwest Airlines, and also fined Frontier Airlines for its continuous delays.
The lawsuit alleges Southwest illegally operated chronically delayed flights and disrupted passengers' travel plans.
Southwest is disappointed that DOT chose to file a lawsuit over two flights that occurred more than two years ago," a spokesperson for the airline said.
The U.S. Transportation Department is suing Southwest Airlines and has fined Frontier Airlines over "chronically" delayed flights, disrupting passengers' travel and plans.
The lawsuit filed in California alleges that Southwest illegally operated chronically delayed flights and disrupted passengers’ travel plans.