WASHINGTON -(Dow Jones)- Twelve oil and gas companies apparently violated U.S. law by using hydraulic-fracturing fluids containing diesel fuel without a permit, congressional investigators said Monday.

From 2005 to 2009, Halliburton Co. (HAL), Baker Hughes Inc.'s (BHI) BJ Services Co., and other firms injected 32.2 million gallons of fluid that contained diesel fuel in wells in 19 states, according to a Jan. 31 letter than Reps. Henry Waxman (D. Calif.), Edward Markey (D. Mass.), and Diana Degette (D. Colo.) sent to Environmental Protection Agency administrator Lisa Jackson.

The EPA has been investigating whether hydraulic fracturing pollutes groundwater and last month accused a producer of contaminating drinking water for the first time. Hydraulic fracturing, or "fracking", involves pumping water and chemicals at high pressures deep underground to extract oil and gas trapped in rock formations.

Lawmakers raised concerns that diesel fuel in the fluids could pollute drinking water supplies.

"If America is to get the benefit of expanded access to domestic natural gas supplies using hydraulic fracturing, key industry players are going to need to improve their environmental performance or they will not be able to convince a skeptical public that this drilling technique will not threaten our drinking water," Markey said in a statement.

The largest users of fluid containing diesel fuel cited by the lawmakers were BJ Services, Halliburton, RPC, Inc. (RES), and Sanjel Corporation. Those companies did not respond to requests for comment.

The Safe Drinking Water Act requires "fracking" companies to obtain water-quality permits when they are injecting fluids underground, but the lawmakers said that EPA has issued no permits authorizing the use of diesel fuel in those wells. No companies have applied for such permits, they said.

"These activities appear to be a violation of the Safe Drinking Water Act because the companies did not obtain permits authorizing the injection of diesel fuel," the lawmakers wrote.

They said they were unable to say how the use of diesel fuel was affecting the environment or public health. They urged Jackson to consider the issue "as part of your investigation into the industry's practices." EPA did not immediately respond to an inquiry about the letter.

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