The Environmental Protection Agency's internal watchdog says there is an ongoing criminal investigation into last year's massive wastewater spill from an abandoned Colorado gold mine.

The EPA's Office of Inspector General sent letters to members of Congress saying it couldn't answer some questions about its review of the Gold King Mine spill without jeopardizing a criminal investigation being conducted with the Justice Department. The statement did not say how long the probe has been underway.

An EPA-led contractor inadvertently triggered the Aug. 5, 2015, spill while doing preliminary cleanup work at the old mine. The resulting 3-million-gallon blowout turned rivers in Colorado, New Mexico and Utah a sickly shade of yellow from wastewater contaminated with toxic heavy metals.

Communities downstream were forced to temporarily halt drawing water from the rivers for drinking water and irrigation. The EPA says water quality in the rivers quickly returned to pre-spill levels.

Several members of Congress had pressed for a criminal investigation into the EPA's role in causing the disaster.

"There's no question as to the EPA's culpability for this spill, and this criminal investigation is critical to ensuring justice is served for all those impacted by this terrible disaster," Sen. John McCain, R-Arizona, said in statement issued Monday.

The EPA is now considering whether to designate the Gold King Mine and other nearby abandoned mines as Superfund sites to help fund more extensive cleanup efforts.