Federal regulators noted the DEQ did not cite any evidence for its claim that small amounts of fracking fluid were used to stimulate the Pavillion field's gas wells — a central tenant of the state's argument that the practice was not to blame for the polluted water.

And while state investigators acknowledged a gap in the protective layer encasing many Pavillion gas wells, they failed to examine whether fracking fluids could have escaped through those openings, the EPA found.

"In summary, the data limitations and uncertainties ... suggest a need for additional investigation to provide support for many of the report's conclusions related to fluid movement, gas source and well integrity," the agency wrote.

EPA's comments came in response to a DEQ report examining the water quality of 13 Pavillion-area drinking wells. The report is one of three produced by the state regarding Pavillion. The others examined well integrity and disposal pits in the area.

A DEQ spokesman said the department had no comment on EPA's review. Wyoming will issue an official response to comments as part of its official investigative process, said Keith Guille.

"We're not going to respond through the media," he said.