Frontier Airlines was slapped with its third fine in three months from the U.S. Department of Transportation.

The latest fine is for long delays on the tarmac after a snowstorm hit Denver last December. The DOT announced last month it would fine Frontier $1.5 million for “violating the Department’s rule prohibiting long tarmac delays.” The fine came with an order to “cease and desist” in future situations.

A DOT investigation concluded Frontier allowed 12 flights at Denver International Airport to sit on the tarmac for more than three hours without allowing passengers an opportunity to deplane. The DOT previously ruled planes with 30 or more seats are prohibited from allowing domestic flights to sit on the tarmac for more than three hours without allowing passengers to get off the flight.

“Frontier failed to properly assess the gate situation during the height of the snowstorm and continued to experience gate availability issues and a ground staff shortage after the storm had passed,” said the DOT in a press release. “Frontier failed to have adequate resources at Denver to accommodate the additional aircraft on the ground at the airport. Moreover, Frontier failed to delay, divert, or cancel a sufficient number of flights scheduled to arrive at Denver, even though it was aware of the conditions at Denver, to allow the carrier to recover and reduce the probability of flights experiencing long tarmac delays. In addition, Frontier could have mitigated or prevented the lengthy tarmac delays if it had accepted services offered by the airport.”

The fine is the second-largest ever levied against an airline for violating the delay rules. Frontier will pay $600,000, as the DOT will credit $900,000 for providing compensation to affected passengers.