US federal regulators have told Southwest Airlines that the carrier failed to provide enough help to travellers who were stranded during massive cancellations last December, and the airline could be subject to a fine. Southwest said in a regulatory filing Monday that it can’t estimate the cost of a fine and damages from lawsuits stemming from nearly 17,000 cancelled flights last December.

Southwest said the US Department of Transportation told the airline Friday that “it has determined the company had failed to provide adequate customer service assistance, prompt flight status notifications, and proper and prompt refunds and that the assessment of a civil penalty is warranted.”

The government indicated in January that it was investigating Southwest over the service collapse, which stranded more than 2 million travellers and cost the airline at least $1.1 billion in lost sales and extra costs including refunds.

Southwest said in a quarterly financial filing Monday that it could also face costs from lawsuits filed by customers and shareholders. The filing followed Southwest’s report that its profit slid 30% in the July-through-September quarter and it will scale back growth plans early next year.