Top Senate Republicans want Attorney General Loretta Lynch to explain why she won't prosecute EPA officials over the Gold King Mine disaster in Colorado.

Sens. John Barrasso, R-Wyo., and John McCain, R-Ariz., wrote in a letter to Lynch and EPA Administrator Gina McCarthy that Lynch's decision to forgo prosecution of EPA officials for the August 2015 toxic mine spill is "unacceptable" and they want Lynch to provide a briefing explaining the decision by Oct. 28.

Barrasso is chairman of the Senate Committee on Indian Affairs and McCain is a top member on the panel.

"This decision shows that the federal government is not interested in holding itself to the same level of accountability to which it holds private companies when their negligence causes serious environmental damage and major economic harm," Barrasso and McCain said in the letter.

The EPA inspector general this month referred criminal findings to Colorado's district attorney for prosecution, but federal prosecutors have decided to instead refer the case to EPA officials for administrative action, McCain and Barrasso said.

The spill contaminated the Animas River that flows between Silverton and Durango, Colo., after EPA workers and EPA contractors damaged the area containing the toxic mine waste, allowing it to spill into the water.

The spill has caused significant harm to Navajo farmland and has caused serious economic and environmental damage to the area.

Barrasso and McCain in the letter question why the EPA will not be held accountable in the same manner as private companies.

The letter cites Duke Energy Corporation, which in 2014 paid a $102 million fine for violating the Clean Water Act after spilling 39,000 tons of coal ash and 27 million gallons of contaminated water into the Dan River in Eden, N.C.

According to the EPA inspector general, EPA officials involved in the Gold King Mine spill also committed violations of both the Clean Water Act as well as violations of the False Claims Act.

"Given the devastation this catastrophe caused in the lives and livelihoods of Native American tribes, including the Navajo Nation, someone needs to be held accountable and legally liable for the largest disaster of its kind since the Deepwater Horizon spill in the Gulf of Mexico," Barrasso and McCain wrote.