(AP)—Internal investigators have faulted the Environmental Protection Agency over years of delays in completing health studies needed to guide the cleanup of a Montana mining town where hundreds of people have died from asbestos exposure.

The EPA's Office of Inspector General says in a report Thursday that the studies are necessary to determine whether expensive, ongoing are working in the town of Libby.

The area near the northwest corner of the state, about 50 miles from the U.S.-Canada border, was declared a in 2009, a decade after federal regulators first responded to concerns over asbestos dust that came from a W.R. Grace vermiculite mine.

The vermiculite was used as insulation in millions of U.S. homes.

At least $447 million has been spent on the cleanup.