PITTSBURGH (AP) -- A new study from the Congressional Government Accountability Office says shale gas and shale oil drilling does pose environmental and public health risks, but the extent of those risks is unknown.

The GAO also says regulators from eight states report the natural gas process called hydraulic fracturing, or fracking, doesn't cause groundwater pollution. The GAO is an independent, nonpartisan agency that works for Congress.

The study says that in some cases a scarcity of scientific data and differing regulations make it difficult to draw firm conclusions about shale oil and gas impacts.

GAO also notes that while federal agencies have given different estimates of the amount of recoverable shale gas in the nation, all the estimates have increased dramatically over the last few years.