A Frontier Airlines plane flies out of Cleveland Hopkins International Airport Wednesday, Oct. 15, 2014.

(AP Photo/Tony Dejak)

Frontier Airlines complained about its Colorado tax bill – now the Denver-based company is being sued by a firm that claims to have helped lower it. 

This all started in 2014, when a Denver Post article caught the attention of a consulting firm called Duff & Phelps. In the article, Frontier said its onerous Colorado tax bill was why it was scaling back operations in the state.

According to their lawsuit, officials with Duff & Phelps alerted Frontier that it was filing its taxes incorrectly, leading the airline to overpay pay by more than $2.5 million. The lawsuit alleges that Frontier used information from Duff & Phelps to fix the tax problem without paying them. Frontier denies the allegations in court documents.

Neither company would comment for this story. The case is pending in Denver District Court.