Washington ? No sign of mad cow disease was found in an animal singled out in preliminary screening last week and then subjected to a follow-up chemical test, the Agriculture Department said Wednesday.

Officials declined to provide additional information about the animal. Test results on a second animal that was possibly being infected will not be available for several days.

“The USDA remains confident in the safety of America’s food supply,” said John Clifford, deputy administrator of the department’s Animal and Plant Health Inspection Service.

Clifford said no meat from the two animals had entered the food supply.

He said tests by the federal laboratory at Ames, Iowa, on tissue from the first animal were negative.

“No further details such as what type of animal, where the animal came from or what lab did the (initial) testing would be disclosed,” he said.

The preliminary test last Friday was the first time in 8,587 such screenings to come back with “inconclusive” results, raising the possibility of a mad cow disease infection. The screenings began June 1.

The same preliminary result was announced Tuesday on a second animal.

There has been only one case of mad cow disease — bovine spongiform encephalopathy, or BSE — in the United States. A sick Canadian-born Holstein was discovered on a farm in Mabton, Wash., in December.

More than 50 countries then cut off imports of U.S. beef and at least 700 additional cattle in Washington state were killed as a precaution.

The disease affects the animal’s brain and nervous system. People who eat products containing the BSE protein can contract a rare but fatal disease similar to BSE, variant Creutzfeldt-Jakob disease. There is no test for mad cow disease except for an examination of the brain and nervous system after an animal is killed.

Japan, one of the major customers for the U.S. beef industry, and many other countries have yet to resume importing American beef.

Negotiations are under way to try to get Japan to end its boycott. The initial screenings on these two animals have raised concern in the industry about the potential effect on the talks, though Clifford sought to play that down.

He said officials have made clear to U.S. trading partners that inconclusive results from the initial testing does not necessarily mean that a cow is infected.

The government plans to screen 268,000 animals in the next 12 months to 18 months. More inconclusive findings can be expected because of the sensitivity of the initial tests, which are designed to cast a wide net to expose potential problems.

European countries have developed some data on cases where initial screening showed a potential problem, but later tests produced a negative finding. Clifford said there had not been enough experience in the United States to make any conclusions.

“We prefer not putting any statistics on that,” Clifford said when pressed on the issue in a conference call with reporters.

Since the Washington state case, the government has barred the use of the most potentially dangerous cattle parts, such as brains and spinal cords, in the food chain.

Even if the disease were discovered in the carcasses, Clifford said the new protections would prevent mad cow from entering the human food supply.

The initial screening targets animals unable to move or showing signs of central nervous system problems and animals that died for unknown reasons. Sample tests also are done on the carcasses of healthy animals.