CASPER, Wyo. (AP) — Rocky Mountain Power says proposed federal environmental regulations could close many of its coal-fired power plants in Wyoming early.

Last month, the U.S. Environmental Protection Agency released a revised version of its plan to control regional haze. The latest plan incorporates some parts of a plan from Wyoming regulators but it includes tougher restrictions for some plants.

Rocky Mountain Power, Wyoming's largest power provider, has four coal-fired plants in the state — Jim Bridger, Dave Johnston, Wyodak and Naughton.

Company spokesman Dave Eskelsen declined to tell the Associated Press which plants which would be endangered by the proposal. He said the company is studying the potential effects on each of its units.