The Environmental Protection Agency's inspector general said it will investigate whether any agency employees involved in the Gold King Mine spill last summer committed any crimes.

The EPA's Office of Inspector General announced it will do a criminal investigation into the spill that polluted the Animas River in Colorado and three southwestern states in August 2015. The criminal investigation will be done at the behest of members of Congress who requested it. It already is conducting a program evaluation into the spill.

In a letter to Rep. Jason Chaffetz, R-Utah, Carolyn Copper, assistant inspector general for program evaluation, said it cannot release details to Congress on its program investigation until the criminal investigation is finished.

"Many of your questions to us, including those that go to the heart of what you asked us to address, directly implicate and will have to be answered in part by investigative results that are not currently releasable," she wrote.

In August 2015, EPA contractors doing cleanup work on the Gold King Mine triggered a toxic spill that polluted the Animas River in Colorado, New Mexico and Arizona. About 3 million gallons of toxic sludge flowed into the river and caused it to turn orange.

The incident has raised the ire of lawmakers and the spill has been the subject of many congressional committee hearings, including ones led by Chaffetz, the chairman of the House Oversight and Government Reform Committee.