ST. LOUIS (AP) — The state environmental agency has denied a permit request to build a $2 billion plant to convert coal to synthetic natural gas in southern Illinois, further clouding prospects for the clean-energy project already more than six years in the making.

The Illinois Environmental Protection Agency told developers Power Holdings LLC in a May 29 letter that the group had not provided required details about emissions, air-quality analysis and other matters involving the proposed plant near Waltonville in Jefferson County.