By MATTHEW BROWNBILLINGS, Mont. (AP) â¿¿ The U.S. Department of Interior is investigating whether mining companies are skirting royalty rules as they increase exports of coal to Asia, federal officials disclosed Friday. No violations have yet been issued, but a newly released report to two U.S. senators says audits of overseas sales have just begun for years when coal export volumes from federal lands grew substantially. The investigation is focused on companies' use of affiliates or brokers to sell coal from mines in the Western U.S. to customers in Asia. The parent company pays government royalties based on the mine price, then the affiliate ships the fuel overseas where it's sold for many times the original price. Interior Secretary Ken Salazar said he has asked the agency's Office of the Inspector General to look into whether such actions violated federal law. He said one federal coal lessee is under investigation for possible criminal violations. Details weren't offered, and his office declined to say if that involved coal shipped overseas. The administration was responding to concerns raised by Sen. Ron Wyden, D-Ore., and Sen. Lisa Murkowski, R-Alaska. They've warned that as coal exports grow, taxpayers could lose many millions of dollars annually if royalties are unfairly calculated. Coal royalties nationwide totaled $876 million last year, from 460 million tons of coal mined from federal lands. Salazar said a special task force of state and federal officials plans to review coal sales and contracts in Montana and Wyoming from 2009 and 2011, and later expand the inquiry to other states for sales dating to 2001. Government agencies typically audit coal sales several years after they have taken place. In a joint statement, Murkowski and Wyden said they were "pleased with the formation of a task force to ensure coal companies have paid their fair share when coal is mined on public lands and sold overseas."