A jar holding waste water from hydraulic fracturing is held up to the light at a recycling site in Midland, Texas. (AP Photo/Pat Sullivan)

This story has been updated.

Science advisers to the Environmental Protection Agency Thursday challenged an already controversial government report on whether thousands of oil and gas wells that rely on hydraulic fracturing, or “fracking,” systemically pollute drinking water across the nation.

That EPA draft report, many years in the making and still not finalized, had concluded, “We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the United States,” adding that while there had been isolated problems, those were “small compared to the number of hydraulically fractured wells.”

The conclusion was widely cited and interpreted to mean that while there may have been occasional contamination of water supplies, it was not a nationwide problem. Many environmental groups faulted the study, even as industry groups hailed it.

But in a statement sure to prolong the already multiyear scientific debate on fracking and its influence on water, the 30-member advisory panel on Thursday concluded the agency’s report was “comprehensive but lacking in several critical areas.”

It recommended that the report be revised to include “quantitative analysis that supports its conclusion” — if, indeed, this central conclusion can be defended.

The panel said its critique was backed by 26 of its members, but four dissented. The advisory group is comprised of academic, government, and industry scientists.

The EPA report in question was originally requested by Congress in 2009, when one of the principal environmental concerns centered on whether fracking could contaminate drinking supplies. Since then, other environmental questions — including concerns over methane emissions from drilling operations — have also gained prominence.

The report, published in June of last year in draft form, represents a nearly five-year effort by the EPA to analyze technical data from thousands from fracking operations and nearby aquifers in states around the country.

The study has been dogged by controversy over the scope and scale of the research. The draft study linked fracking to a few cases of water pollution but said the problems appeared so far to be isolated. It cautioned that a number of fracking-related activities carry a future risk of polluting wells and aquifers used for drinking and farming.

The EPA acknowledged last year that it was hampered in its assessment by inadequate data, preventing experts from reaching firm conclusions about whether contaminants in an individual well came from fracking or another source. Some critics of the report said poor data skewed the agency’s conclusions.

The advisory panel’s evaluation and critique of the draft study was wide ranging, but focused in closely on the assertion that fracking operations had not led to “widespread, systemic impacts on drinking water resources.” The committee said that this conclusion had not been backed up “quantitatively” by the agency, adding that the report “did not clearly describe the system(s) of interest (e.g., groundwater, surface water), the scale of impacts (i.e., local or regional), nor the definitions of ‘systemic’ and ‘widespread.'”

This goes to the very heart of the issue, because it’s one thing if, occasionally, there have been some unfortunate accidents — but another if there is something inherent to the entire process of unconventional gas development that harms drinking water.

The four dissenting members of the advisory panel, however, countered that the report’s statement about the lack of “widespread, systemic impacts” was ” clear, unambiguous, concise, and does not need to be changed or modified.”

“While the report could have articulated the agency’s statistical assessment more clearly, there has not been any facts or evidence demonstrating a systemic or widespread impact to existing drinking water resources or other water resources,” they argued.

The Sierra Club hailed the Science Advisory Board critique, saying it “called out” the agency. “Instead of blindly allowing destructive fracking to continue in our communities, we should extend statewide fracking bans and moratoriums that will keep dirty, climate-polluting fossil fuels like fracked gas in the ground.”

However, the American Petroleum Institute released a statement Friday challenging the Scientific Advisory Board.

“The science is clear and the studies are completed,” said Erik Milito, who is director of upstream and industry operations for the group. “Study after study shows that hydraulic fracturing is safe.”

“Instead of denying the scientific evidence proving the environmental benefits of hydraulic fracturing, the United States should be celebrating the overwhelming data demonstrating that hydraulic fracturing is helping reduce GHG emissions and other emissions, and has helped lower energy costs for consumers,” Milito continued.

Hydraulic fracturing, or fracking, refers to oil and gas wells that are drilled deep in the ground and then horizontally. After that, water containing some chemicals is blasted at high pressure into the well to fracture the rock, unlocking previously trapped natural gas or oil that then flows back up the well.

This technology has driven a major domestic U.S. oil and gas boom that has contributed to the recent plunge in oil prices and triggered the collapse of the coal industry. But there have also been widespread claims that major fracking operations in some communities have contaminated drinking water with methane gas or other substances.

“EPA will use the [science advisory board’s] final comments and suggestions, along with relevant literature published since the release of the draft assessment, and public comments received by the agency, to revise and finalize the assessment,” agency spokeswoman Melissa Harrison said in a statement. “EPA appreciates the work done by the SAB and hopes to finalize the assessment in 2016.”