WASHINGTON — The inspector general for the Environmental Protection Agency is investigating the cause of a massive spill from an abandoned Colorado gold mine that unleashed 3 million gallons of contaminated wastewater into rivers that supply water to at least three states.

The inspector general's office said the investigation also will focus on the EPA's response to the Aug. 5 spill from the defunct Gold King Mine near Silverton, Colo. About 3 million gallons of water containing heavy metals such as arsenic, cadmium, lead and mercury were released into a tributary of the Animas River, turning the river sickly yellow and raising concerns about long-term environmental damage.

A diluted toxic plume reached Lake Powell, a huge reservoir 300 miles downstream that feeds the Colorado River and supplies water to the Southwest.