The government has ordered a definitive laboratory test after two preliminary screening tests raised the possibility of mad cow disease in the tissue of a recently slaughtered animal, federal officials said yesterday.

A conclusive answer could take as long as a week. Two other cases that were considered suspicious following initial screening tests in June later proved to be harmless. In those cases, however, the preliminary tests were run only once on each sample.

The new case sent jitters through the beef industry, which was severely hit after the discovery of the first American case of mad cow disease in Washington state about a year ago.

The location of the newly identified animal was not made public because of the preliminary nature of the results, but officials said the meat had not entered the food supply or been used for animal feed. Nevertheless, investigators have begun tracing the animal's origins to be prepared to locate any potential source of contamination and, potentially, other infected animals if the infection is confirmed, officials said.

Government and industry officials insisted that the suspicious or "inconclusive" result on two screening tests did not make it more likely that the animal was infected. But David Ropeik, director of risk communication at the Harvard Center for Risk Analysis, said the two preliminary tests do raise the odds that this animal would be confirmed as a positive case.

"If it only comes back once, it is not as suggestive as if it comes back twice," he said. "This increases the likelihood this cow may be positive, but it is not like a sure thing."

In August, the Department of Agriculture changed its standard and decided it would require two suspicious results on screening tests before seeking the more sophisticated, definitive test. In making the change, officials said they were following the test manufacturer's instructions.

The animal identified yesterday was the first deemed suspicious since the change in procedure, but federal officials said yesterday that there had been other cases in which screening tests had produced a single suspicious finding. In every case, duplicate tests showed no problem. None of those tissue samples was referred for the more sophisticated test, and none of those cases was made public.

Andrea Morgan, associate deputy administrator at the Agriculture Department, would say only that such cases had occurred "infrequently." The results are now considered "lab records," she said, and not public information.

Harvard's Ropeik said that even if a new case of mad cow disease is discovered, the risk to the public is essentially zero because of safeguards that have been in place for several years. Mad cow disease, or bovine spongiform encephalopathy (BSE), can cause a fatal, brain-wasting disease in humans who consume beef from infected animals.

The gold-standard test will be conducted at the National Veterinary Services Laboratories in Ames, Iowa.

"I wouldn't view this as any different than what we've seen before," said Gary Weber, executive director of regulatory affairs at the National Cattlemen's Beef Association.

Both Weber and the USDA's Morgan stressed that inconclusive results were a product of using a very sensitive test: "The inconclusive result does not mean we have found another case of BSE in this country," she said in a statement. "Inconclusive results are a normal component of screening tests, which are designed to be extremely sensitive so they will detect any sample that could possibly be positive."

Caroline Smith DeWaal, food safety director for the Center for Science in the Public Interest, a public advocacy group, said the Bush administration had been slow to implement changes it promised last year, after the detection of a Holstein cow infected with mad cow disease derailed virtually all American beef exports.

She said the nation needs a national identification system that can track all cattle back to their farm of origin, the Food and Drug Administration should finalize new safety rules and the USDA should be given the authority to issue mandatory recalls of tainted meat, rather than relying on voluntary cooperation from industry.

Wayne Pacelle, chief executive of the Humane Society of the United States, said he was concerned that with the departure of Agriculture Secretary Ann M. Veneman, who announced her resignation this week, safety policies announced last year might be weakened. "This is an important reminder that we cannot relax in our efforts to see that BSE-positive animals don't get into the food supply," he said.