The senator made the declaration Friday during an Indian Affairs Committee field hearing in Phoenix, Ariz. The hearing was titled: "Examining the Environmental Protection Agency's (EPA) Unacceptable Response to Indian Tribes."

"I've come to the conclusion that a Department of Justice criminal investigation is merited and must now occur," McCain said.

He cited EPA employees' failure to conduct appropriate water pressure tests at the mine and adequately consult partnering federal agencies before excavating around the mine's containment plug, though they knew about the potential for a "catastrophic blowout.”

The spill "has devastated lands and livelihoods across Navajo Nation" McCain said in a statement. He said he is concerned about the EPA's "disregard for environmental issues in Indian Country."

The waste water spill at the abandoned Gold King Mine in August of 2015 dumped 3 million gallons of toxic sludge into the Animas River in Colorado, tainting waterways of three states. A report by the Interior Department in October of 2015 concluded that had the EPA properly conducted water pressure tests at the mine, the crisis could have been averted.

A House Natural Resources Committee report in February said the EPA tried to conceal negligence that lead to the spill.

McCain and Indian Affairs Chairman John Barrasso subpoenaed EPA officials to testify at the hearing. Mathy Stanislaus, EPA assistant administrator for the Office of Land and Emergency Management, appeared along with Arizona Rep. Ann Kirkpatrick and Navajo and Hopi tribal leaders.