In opening an inquiry in Parker County, Texas, in late 2010, the EPA examined a question similar to the one it faced in Dimock: Was a driller responsible for methane gas bubbling up in residents' water wells?

This time, though, tests conducted by a geologist hired by the agency appeared to confirm that the methane in the wells had resulted from drilling, rather than occurring naturally.

"The methane that was coming out of that well … was about as close a match as you are going to find," said the consultant, Geoffrey Thyne, a geochemist and expert in unconventional oil and gas who has been a member of both the EPA's Science Advisory Board for hydraulic fracturing, and a National Research Council committee to examine coalbed methane development.

The EPA issued an "imminent and substantial endangerment order" forcing Range Resources, the company it suspected of being responsible, to take immediate action to address the contamination.

But once again, the EPA's actions ignited an explosive response from the oil and gas industry, and a sharp rebuke from Texas state officials, who insisted that their own data and analysis proved Range had done no harm.

According to the environmental news site Energy Wire, Ed Rendell, the former Governor of Pennsylvania, whose law firm lobbies on behalf of energy companies, also took up Range's case with then-EPA Administrator Lisa Jackson.

Internal EPA emails used in the EnergyWire report and also obtained by ProPublica discuss Rendell's meeting with then-EPA Administrator Lisa Jackson, though Range has denied it employed Rendell to argue on its behalf. Neither the EPA nor Rendell responded to a request for comment on the Parker County case.

In March 2012, the EPA dropped its case against Range without explanation. Its administrator in Texas at the time had been assailed for making comments that seemed to show an anti-industry bias. He subsequently lost his job. An Associated Press investigation found that the EPA abandoned its inquiry after Range threatened not to cooperate with the EPA on its other drilling-related research.

Agency critics see a lack of will, rather than a lack of evidence, in the EPA's approach in Parker County and elsewhere.

"It would be one thing if these were isolated incidents," said Alan Septoff, communications director for Earthworks, an environmental group opposed to fracking. "But every time the EPA has come up with something damning, somehow, something magically has occurred to have them walk it back."