WASHINGTON — Five members of Congress called Thursday for investigations into the Bush administration's regulation of hydraulic fracturing, an oil and gas drilling technique pioneered by Halliburton Co., Vice President Dick Cheney's former employer.

In separate requests, the lawmakers urged the Environmental Protection Agency's inspector general to investigate the practice, called for congressional hearings and submitted detailed questions to EPA Administrator Mike Leavitt. Four of the lawmakers are Democrats; the fifth is an independent.

"We must do all that we can to make sure that this hydraulic fracturing activity is done safely," Reps. Mark Udall and Diana DeGette, both Democrats from Colorado, said in a letter to the EPA's inspector general requesting an inquiry. "This is particularly important because water is a scarce resource, especially in this prolonged drought."

Hydraulic fracturing provides access to hard-to-reach oil and gas deposits by pumping liquids underground at high pressure. The liquids sometimes include diesel fuel or other hazardous chemicals. Some of the fluids remain in the ground.

Halliburton, which Cheney headed from 1995 until 2000, is one of three U.S. companies that dominate the fracturing market. The Houston-based company generates about $1.5 billion a year in revenue from fracturing.

Sens. James M. Jeffords (I-Vt.) and Barbara Boxer (D-Calif.), members of the Senate Environment and Public Works Committee, asked Leavitt to explain why the agency was not monitoring fracturing nationally despite a federal court decision mandating regulation under the Safe Drinking Water Act.

Rep. Henry A. Waxman (D-Los Angeles) also wrote to the inspector general, asking whether "political considerations improperly influenced" an EPA study completed in June that found that fracturing in coal bed methane fields did not threaten drinking water supplies. Waxman and DeGette are on the House Energy and Commerce Committee.

Waxman cited a report in Thursday's Times that disclosed that EPA employees had criticized the study before its completion and after its publication. One of the employees, Weston Wilson, an environmental engineer in the EPA's Denver office, last week sent an 18-page statement challenging the study's conclusions and methodology to the agency's inspector general and to the Colorado congressional delegation.

Wilson criticized the failure to include field research, and the EPA's selection of a panel heavily weighted toward the energy industry to review the study.

An EPA spokeswoman, Cynthia Bergman, said the agency was reviewing Wilson's letter, but added, "We do not believe that any of the concerns raised by his analysis would lead us to a different conclusion."

The office of EPA Inspector General Nikki L. Tinsley had received Wilson's statement and was reviewing it, a spokesman said Thursday.

The EPA study found that although some of the fracturing fluids were hazardous when undiluted, they were sufficiently watered down before injection to minimize the risk to sources of drinking water. The study found no proof that fracturing directly caused contamination.

The controversy over regulation of fracturing dates back a decade. Following years of litigation, a federal appeals court in Alabama ruled in 1999 that the EPA was required to regulate the practice in the same way it regulates injection wells used for disposal of drilling waste.

That decision sparked an industry lobbying campaign to pass legislation exempting fracturing from regulation under the Safe Drinking Water Act. Halliburton, which controls at least one-third of the domestic fracturing operations, has said that regulation would increase costs and delays and have a severe effect on its business.

The Alabama decision has not been applied nationally by the EPA. In their letter, Jeffords and Boxer asked Leavitt to explain why the agency had not done so.

DeGette and Udall urged the EPA's inspector general to investigate Wilson's allegations.

"People's health and safety as well as overall environmental quality may be at risk" if fracturing proceeds without adequate controls, the lawmakers wrote.

In interviews, DeGette and Udall said they would seek congressional hearings on fracturing and the possible need for more regulation and oversight.

"It would serve us well to get a clear picture of what is actually occurring with this technology," Udall said. "I am not against responsible oil and gas development, but if we are creating a long-term problem by polluting precious groundwater, it is not worth the short-term gain."