CHEYENNE, Wyo. (AP) - U.S. Sen. Jim Inhofe (R-OK)is asking the head of the U.S. Environmental Protection Agency for more information about an EPA investigation into groundwater contamination in a central Wyoming gas field.

The ranking Republican on the Senate Environment and Public Works Committee wrote EPA Administrator Lisa Jackson on Tuesday and asked her to explain a recent comment she made about the contamination. Jackson said that hydraulic fracturing could have affected areas containing groundwater.

Hydraulic fracturing is pumping pressurized water, sand and chemicals underground to open fissures and improve the flow of gas. Environmentalists say "fracking" is a threat to groundwater but the petroleum industry says there isn't a single documented case of fracking contaminating groundwater. Inhofe asked Jackson about the basis for her comment.

The EPA says it's reviewing the letter.