The government hopes DNA analysis can pinpoint the herd of the cow that tested positive for mad cow disease and lead investigators to the source of the animal's brain-wasting illness, the Agriculture Department's chief veterinarian said yesterday.

Genetic testing is needed because of mistakes in how the beef cow was labeled and how its tissues were stored, veterinarian John Clifford said.

The cow, a "downer" that could not walk, was delivered in November to a plant where animals unfit for human consumption are killed. The department has not identified the owner or the plant.

The cow's breed was mislabeled, possibly because the animal had been soiled heavily with manure and because its tissues were mixed with tissues from other cows, Clifford said.

"When we went back to this particular owner, the breed we identified, he indicated he did not sell that breed. He sold another breed," Clifford said. "In addition to that, we found that after the tissues were processed, there was some mixing."

Parts from the diseased animal and four other cows were supposed to be kept in separate waste barrels, but some of the waste was combined, Clifford said.

Department officials said they think they have found the right herd. To confirm that, they must find relatives of the dead cow and test DNA.

"We're pretty confident that we have the herd, but we want to make sure," Clifford said. "Testing is being done now on tissue from cows that may have been herdmates."

Finding the herd will help track the cow's feed and explain how the animal became infected. Mad cow disease is known to be spread through the feeding of infected cattle remains to other cattle. The United States banned the practice of using cattle remains in animal feed in 1997.

It may be the first native-born case of mad cow disease. Agriculture Secretary Mike Johanns said there is no evidence the animal was imported. The only other U.S. case, confirmed in December 2003 in Washington state, was in a dairy cow that had been imported from Canada, where three other cases have been found.

The 2003 case prompted nearly 50 countries to impose bans on American beef, causing billions of dollars in losses for the U.S. industry. Hours after officials confirmed the new case, Taiwan reimposed its ban. Japanese officials said they would seek more information about the new case.