"The rigor with which we're undertaking efforts to protect public health and the environment have not been affected by these delays," he said.
But the investigators said poor communication with Libby residents, members of Congress and the Inspector General's Office compounded the problem. They added that the agency's lack of transparency could undermine confidence in its work in Libby.
A draft toxicology study that is key to completing the risk assessment for Libby says even an extremely small amount of asbestos fibers from the now-shuttered W.R. Grace mine can cause health problems.
Representatives of W.R. Grace and others in the chemical industry have pushed for revisions, saying the toxicity level set by the EPA is impractical because it exceeds background levels of asbestos found in some parts of the country.
Montana U.S. Sen. Max Baucus said in a statement responding to Thursday's report that the EPA needs to avoid its past mistakes and get its studies done quickly.
"We need to move forward with this toxicological assessment, so we are making the right decisions based on the right science," Baucus 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 if the agency's studies determine certain properties need to be revisited.
Work on the mine site outside town has barely begun. It closed in 1990 and remains the responsibility of W.R. Grace. A company spokesman did not immediately reply to an Associated Press request for comment.