WASHINGTON - The Bush administration is undermining the Environmental Protection Agency's ability to determine health dangers of toxic chemicals by letting nonscientists have a bigger — often secret — role, congressional investigators say in a report obtained by The Associated Press.

The administration's decision to give the Defense Department and other agencies an early role in the process adds to years of delay in acting on harmful chemicals and jeopardizes the program's credibility, the Government Accountability Office concluded.

At issue is the EPA's screening of chemicals used in everything from household products to rocket fuel to determine if they pose serious risk of cancer or other illnesses.

A new review process begun by the White House in 2004 is adding more speed bumps for EPA scientists, the GAO said in its report, which will be the subject of a Senate Environment Committee hearing Tuesday. A formal policy effectively doubling the number of steps was adopted two weeks ago.

Cancer risk assessments for nearly a dozen major chemicals are now years overdue, the GAO said, blaming the new multiagency reviews for some of the delay. The EPA, for example, had promised to prepare assessments on 10 major toxic chemicals for external peer review by the end of 2007, but only two reached that stage.

GAO investigators said extensive involvement by EPA managers, White House budget officials and other agencies has eroded the independence of EPA scientists charged with determining the health risks posed by chemicals.

The Pentagon, the Energy Department, NASA and other agencies — all of which could be severely affected by EPA risk findings — are being allowed to participate "at almost every step in the assessment process," said the GAO.

Those agencies, their private contractors and manufacturers of the chemicals face restrictions and major cleanup requirements, depending on the EPA's scientific determinations.

"By law the EPA must protect our families from dangerous chemicals," said Sen. Barbara Boxer, D-Calif., the Senate committee's chairman. "Instead, they're protecting the chemical companies."

The EPA's risk assessment process "never was perfect," Boxer said in an interview Monday. "But at least it put the scientists up front. Now the scientists are being shunted aside."

The GAO said many of the deliberations over risks posed by specific chemicals "occur in what amounts to a black box" of secrecy because the White House claims they are private executive branch deliberations.

Such secrecy "reduces the credibility of the ... assessments and hinders the EPA's ability to manage them," the GAO report said.

The White House said the GAO is wrong in suggesting that the EPA has lost control in assessing the health risks posed by toxic chemicals.

"Only EPA has the authority to finalize an EPA assessment," Kevin F. Neyland, deputy administrator of the White House budget office's Office of Information and Regulatory Affairs, wrote in response to the GAO. He called the interagency process "a dialogue that helps to ensure the quality" of the reviews.

One EPA scientist with extensive knowledge of the changes in the agency's risk assessment policies ridiculed the claim that the EPA still has the final say.

"Unless there is concurrence by other agencies, ... things don't go forward. It means we stop what we are doing," said the scientist, speaking on condition of anonymity because of fear of endangering his career.

"The (EPA) scientists feel as if they have lost complete control of the process, that it's been taken over by the White House and that they're calling the shots," the scientist said.

The GAO investigation focused on the EPA's computerized database, known as IRIS — the Integrated Risk Information System. It contains data on the human health effects of exposure to some 540 toxic chemicals in the environment. New chemicals are being proposed constantly for inclusion under a complicated assessment process that can take five years or more.

After years of stops and starts, the GAO said, the EPA has yet to determine carcinogen risks for a number of major chemicals such as:

_Naphthalene, a chemical used in rocket fuel as well as in manufacturing commercial products such as mothballs, dyes and insecticides.

_Trichloroethylene, or TCE, a widely used industrial degreasing agent.

_Perchloroethylene, or "perc," a chemical used in dry cleaning, metal degreasing and making chemical products.

_Formaldehyde, a colorless, flammable gas used to making building materials.

Environmentalists say these chemicals have been widely found at military bases and Superfund sites and in soil, lakes, streams and groundwater.

The findings, after an 18-month investigation by the congressional watchdog agency, come at a time of growing criticism from members of Congress and health and environmental advocates over alleged political interference in the government's science activities.

Last week, a confidential survey by an advocacy group of EPA scientists showed more than half of the 1,600 respondents worried about political pressure in their work.