Coal sales in western states are under increased scrutiny from lawmakers after revelations of problems including reserves of the fuel sold at prices below market value.
A letter from the U.S. Department of Interior Inspector General released Friday shows federal officials in Colorado, Utah, Wyoming and New Mexico accepted below-market bids for coal or sold the fuel without full appraisals.
Inspectors said that violated federal rules including the 1920 Mineral Leasing Act, which requires coal sales to be competitive.
The letter was released by U.S. Sen. Ron Wyden, an Oregon Democrat who has been critical of the government's coal program.
A Government Accountability Office report earlier this week found other problems with coal sales that Massachusetts U.S. Sen. Ed Markey says might have cost taxpayers hundreds of millions of dollars.