WASHINGTON -- Three House Democrats are pressing the U.S. Environmental Protection Agency to require oil and gas companies to disclose the fluids they use to hydraulically fracture wells.

Reps. Henry Waxman of California, Ed Markey of Massachusetts and Diana DeGette of Colorado earlier this year released an analysis showing that 12 oil and gas companies had used 32.2 million gallons of fracturing fluid containing diesel fuel between 2005 and 2009.

Fracking involves injecting mixtures of water, sand and chemicals at high pressure deep underground to break up shale-rock formations and make it easier to extract oil and natural gas.

The lawmakers wrote the EPA this week that they are revising that number upward by 500,000 gallons because two companies had accidentally provided lawmakers incorrect data. The errors "demonstrate the difficulty in obtaining accurate information about the contents of hydraulic fracturing fluids and reinforce the need for mandatory and uniform national disclosure of this information to EPA," the lawmakers wrote.

Oil and gas companies have long maintained that the process doesn't threaten groundwater, but some Democrats and environmentalists say it has caused several instances of drinking-water contamination with toxic chemicals such as benzene, a known carcinogen.

The EPA is conducting its own study on the safety of hydraulic fracturing. The agency recently announced that it would set standards for the disposal of fracturing wastewater.

Texas, where companies will have to disclose their fracturing fluids' ingredients under a new state law, led the way among states with 16.7 million gallons of diesel-containing fluid, more than half the total, according to the Democratic lawmakers.

Louisiana adopted its own disclosure-requirement rule this month.

Ms. DeGette is lead sponsor of a bill to require companies to disclose the fluids they use for fracturing. The bill, introduced in March, hasn't been brought up for consideration in the Republican-controlled House.

Interior Secretary Ken Salazar earlier this month said his department was drafting a disclosure-requirement rule for fracturing on public lands. An Energy Department advisory board is also working to finalize a report from August with recommendations on fracturing safety.