A routine test indicated the possible presence of mad cow disease, said John Clifford, the USDA official. The agency would not say where the animal was from.
The cow did not enter the human or animal food chain, Clifford said.
The department is conducting more detailed tests at its laboratory in Ames, Iowa, and should have results in four to seven days.
"This inconclusive result does not mean we have found a new case of BSE," Clifford said, giving the abbreviation for the disease's formal name, bovine spongiform encephalopathy.
"Inconclusive results are a normal component of most screening tests, which are designed to be extremely sensitive," he added in a statement.
In humans, eating meat products contaminated with mad cow disease has been linked to more than 150 deaths worldwide from variant Creutzfeldt-Jakob Disease, a rare and fatal nerve disease.
A majority of the deaths were in Britain, where there was an outbreak of mad cow disease that started in the mid-1980s. There was one case confirmed in the U.S., although the federal Centers for Disease Control believes the person got the disease while in the United Kingdom.
No one is known to have contracted the disease inside the United States.
U.S. government investigators have found two cases of mad cow disease. The first was in December 2003 in a Canadian-born cow in Washington state. The second was last June in a cow that was born and raised in Texas.
In response to the first case, the Agriculture Department increased its level of testing for the disease. As of Friday, 644,603 of the nation's estimated 95 million head of cattle had been tested.
Tests are done on dead animals; there is no test for the disease in a live cow. The department primarily tests animals that can't walk, have signs of nervous system disorder, are emaciated or injured or that have died. These animals are considered to be at greatest risk of having the disease.
Clifford said the U.S. has "a system of interlocking safeguards" against mad cow disease that protects people and health. The U.S. has a ban on adding remains of dead cattle to feed for live cattle, because eating contaminated feed is how the disease is believed to spread.
The government also requires the removal of tissues known to carry the disease, such as brains and spinal cords, when animals are slaughtered.
"We remain very confident in the safety of U.S. beef," Clifford said.