The officials released few details and refused to say where the possibly diseased animal was found. They said it would be four to seven days before more could be confirmed.
John McBride of the Livestock Marketing Association told CBS News Correspondent Barry Bagnato the delay will cripple the beef markets for the next several days until the final tests are in.
But, he said, "after Americans eat their turkey, they should be able to slap a steak on the grill."
Mad cow disease, or bovine spongiform encephalopathy, attacks an animal's nervous system. People who eat food contaminated with BSE can contract a rare disease that is nearly always fatal, variant Creutzfeldt-Jakob disease.
The possible case comes 11 months after the United States had its first case of mad cow disease. Japan and other countries are still maintaining bans against U.S. beef as the result of the earlier case.
Suspicions about another case of the disease came because of an inconclusive test result, officials said.
"The inconclusive result does not mean we have found another case of BSE in this country," said Andrea Morgan, associate deputy administrator of the USDA's Animal and Plant Health Inspection Service.
She said the 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."
"It is important to note that this animal did not enter the food or feed chain," Morgan said. "USDA remains confident in the safety of the U.S. beef supply. Our ban on specified risk materials from the human food chain provides the protection to public health, should another case of BSE ever be detected in the United States."
Morgan said initial efforts had begun to trace back the animal from where it was tested to the farm where it originated from.
The wait to find out more about this possible new case of BSE has "put the entire industry really in limbo," said John McBride, a spokesman for the Livestock Marketing Association.
"With final results not being available for four to seven days, it's going to disrupt the livestock market. Buyers are going to be reluctant to buy, sellers are going to be reluctant to put their livestock on the market," he said. "The effect on the market could be profound."
Officials at the Cattlemen's Beef Promotion and Research Board and monitors consumer perceptions and attitudes, had no immediate comment.
Just before the start of the July Fourth holiday weekend, the department had announced two other possible cases of the brain-wasting illness in the United States — but then said follow-upu testing had proved negative. Both were subjected to the more definitive testing after initial screenings for infection were inconclusive.
Thousands of animals have been tested under new screening procedures that took effect June 1 to address complaints that too few animals in the United States are tested for the disease. The mad cow screening programs used by the government were developed by Bio-Rad Laboratories and have been used in Europe for a number of years.
In the only confirmed U.S. case, a Canadian-born Holstein was found to have been infected, but just that one case caused Japan and more than three dozen other countries to refuse U.S. beef. That hurt U.S. export sales and the farm economy.
Bush administration officials are now focused on trying to get those bans lifted and with establishing a national identification system for tracking livestock and poultry from birth through the production chain.
Such a system has worried producers who prefer to keep their records confidential or run a voluntary ID clearinghouse that would provide government officials with limited access.