Hydraulic fracturing, or fracking, does not lead to widespread or systematic contamination of drinking water resources, the U.S. Environmental Protection Agency (EPA) said Thursday, although the agency acknowledged that its conclusion could have been affected by a lack of available data.

The EPA identified key areas of vulnerability in the fracking process for its draft report on the practice’s potential effects on drinking water resources. But it said that, based on available data, fracking did not lead to systematic contamination.

“We did not find evidence that these mechanisms have led to widespread, systematic impacts on drinking water resources in the U.S.,” the report said, adding that the finding was based on the low number of contamination cases compared to number of wells.

The EPA report said the finding “could reflect a rarity of effects on drinking water resources” — or it could simply be the result of “insufficient pre- and post-fracturing data on the quality of drinking water resources.”

Fracking, which became widespread in the United States in the early 2000s, has transformed oil and gas extraction technology, but the growth in domestic drilling has raised concerns over potential health and environmental impacts. Fracking is a method of extracting oil and natural gas from shale rock by injecting highly pressurized fluids and chemicals into the ground.

An estimated 25,000 to 30,000 wells have been drilled and fracked every year in the U.S. between 2011 and 2014 — mostly concentrated in Texas, Colorado, Pennsylvania, and North Dakota — the EPA report said. About 6,800 sources of drinking water, serving more than 8.6 million people, are located within one mile of a fracking well, the report added.

The EPA estimated the number of spills already taking place across the U.S. could range from 100 to 3,700 a year.

It said available data showed that in Colorado there was an average of one spill for every 100 wells each year, and between 0.4 and 12.2 for every 100 wells in Pennsylvania.

More than 1,000 chemicals are reported by industry in fracking fluids, and their ability to reach and affect a drinking water source varies, the EPA said, adding that many of the chemicals tend to remain in water long-term.

The data used for the EPA's report included information from “relevant scientific literature and data,” federal and state government reports, nongovernmental organization reports and publications submitted to the EPA by the oil and gas industry, the report said.

Environmentalists blasted the report, saying the study fell short of the level of government oversight needed to protect the health and safety of millions of Americans living near fracking sites.

“The EPA found disturbing evidence of fracking polluting our water despite not looking very hard,” Kassie Siegel, of the Center for Biological Diversity, said in an emailed statement. “This study was hobbled by the oil industry’s refusal to provide key data.”

She was referring to reports that the EPA had been struggling in recent years to collect the necessary baseline data on drinking water resources to be able to determine whether the quality of the water was impacted by fracking.

Inside Climate News reported in March that the EPA was not able to legally force cooperation by fracking companies, almost all of which refused when the EPA asked them to participate in these baseline studies. Two companies eventually agreed to take part, but one fell through, the news website added.

One of the companies, Chesapeake Energy, "chipped away at the scope of the plan over two years of talks, limited when and where the EPA could monitor water," Inside Climate News reported.

After years of attempting to get the oil and gas industry on board for the studies, the EPA had to continue with the report without that data or risk the overall study, according to Inside Climate News. The study, originally due in 2012, is now planned for a 2016 release.

The EPA study, commissioned by Congress in 2010 in a bid to find a definitive answer on fracking’s impact on drinking water, has “industry’s oily fingerprints all over it,” according to Wenonah Hauter, executive director of Food & Water Watch, a Washington, D.C.-based consumer rights group.

“Sadly the EPA study released today falls far short of the level of scrutiny and government oversight needed to protect the health and safety of the millions of American people affected by drilling and fracking for oil and gas,” Hauter said in an email.

Tom Burke, science adviser and deputy assistant administrator of the EPA’s Office of Research and Development, said in a conference call Thursday that the study was meant to be a foundation for future decisions and represents the “most comprehensive assessment to date of the scientific data available.”

Burke said the report is not comprehensive, and because the lack of data could affect the EPA’s findings, the report is meant to be “moving science forward” rather than giving a final answer on the potential of fracking contamination.

The study identified vulnerabilities in the fracking process that could contribute to spills, Burke said. He said they included water withdrawals from areas with low water availability; fracking into geologic formations containing drinking water; inadequately cased or cemented wells resulting in the below-ground movement of gases and liquids; inadequately treated wastewater discharged into drinking water; and spills.

The EPA said that its regulatory authority is limited by exemptions under the Clean Water Act and the Safe Drinking Water Act, and that states have the responsibility of playing the primary regulating role in oil and gas development.

But state regulators may lack access to the same data that federal agencies have, a recent interview with California regulators showed.

In December, Al Jazeera spoke with California’s groundwater monitoring chief, John Borkovich, for its story entitled, "Californians learning that dwindling water supply contaminated," just before it came to light that oil companies in the state had drilled more than 170 disposal wells — which store used fracking fluid — into aquifers with water suitable for drinking.

When asked if Borkovich was sure that the oil and gas activities were not poisoning millions of gallons of California’s water, he said, “I don’t know.”

The EPA said it was “shocked” that state regulators had erred in allowing drilling into aquifers that held drinking water resources, the LA Times reported. Some state lawmakers — including Sen. Hannah-Beth Jackson, D-Santa Barbara, who called the regulators' failings “endemic” — characterized regulatory agency practices in the state as inept and corrupt, the Times said.

This example and the lack of cooperation between industry and the EPA for its study has angered environmentalists, and left many wondering who — if not the federal or state agencies — is regulating fracking in the U.S.

“The U.S. Environmental Protection Agency's study contorts facts to cover for the oil and gas industries, much like Gov. Brown has been doing in California,” said Rose Braz, climate campaign director for the Center for Biological Diversity. “But even with its limited scope, the study confirms that oil extraction endangers our fragile water supply.”