Internal investigators faulted the Environmental Protection Agency on Thursday 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 said in a report that the studies are necessary to determine whether expensive, ongoing cleanup efforts 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 public health emergency 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, and the town remains under a first-of-its kind public health emergency declaration issued by then-EPA administrator Lisa Jackson in 2009. The deaths are expected to continue for decades due to the long latency of asbestos-related diseases.

For his part, EPA Acting Regional Administrator Howard Cantor said the agency strongly disagrees with much of the Inspector General's conclusions.

Cantor said the risk and toxicity studies are complex endeavors that need to be done properly to make sure Libby's residents are protected.

He added that the cleanup has already addressed 1,700 homes and commercial properties and 1.2 million tons of contaminated soil have been removed.

"The rigor with which we're undertaking efforts to protect public health and the environment have not been affected by these delays," he said.

Meanwhile, the cleanup grinds on. At least 80 and up to 100 properties in town are queued up for work this year, according to the EPA.

Several hundred properties still need to be addressed, and that list could grow significantly.

Work on the mine site outside town has barely begun. It closed in 1990 and remains the responsibility of W.R. Grace.