BILLINGS, Mont. (AP) — Congressional investigators have found problems with federal coal sales that a leading lawmaker says might have cost taxpayers $200 million or more in lost revenue.

U.S. Sen. Ed Markey of Massachusetts says his office determined public coal sales could have yielded the additional revenue and possibly much more. Markey says the sales should be suspended pending reforms.

A sweeping examination of the Interior Department's coal-leasing program was released Tuesday by the U.S. Government Accountability Office.

It shows investigators uncovered widespread inconsistencies in how the government values public coal reserves that are leased to private mining companies.

More than 40 percent of U.S. coal production comes from public lands. Exports to lucrative Asian markets have surged in recent years, stirring concern that companies could be shortchanging taxpayers.