The government has killed and is testing 29 cows from the herd of the Texas cow infected with mad cow disease, the Agriculture Department (search) said Friday.

Investigators have been working to identify offspring and herd mates born within a year of the infected cow's birth. The infected cow was a 12-year-old Brahma (search) cross beef cow.

Twenty-nine adult cows were removed from the herd on Wednesday, the department said. They were taken to a collection site and euthanized, and tissue samples were removed for testing, the department said.

The samples will undergo screening.

Rapid tests indicated the presence of mad cow disease in the animal, but results from a different test were negative, and the department announced the animal was free of the brain-wasting disease. Last month, the agency's internal watchdog ordered a third type of test that came back positive, and a laboratory in England (search) confirmed the results on June 24.

The U.S. changed its testing policy as a result of the new case. Now, both additional tests — immunohistochemistry and Western blot — will be done if rapid tests indicate a sample is suspect.

The Texas animal is the first native case of mad cow disease. The other U.S. case, confirmed in December 2003, was in a dairy cow in Washington state that had been imported from Canada, where investigators believe it ate contaminated feed.

Also under investigation is the feed history of the Texas herd, because the only way mad cow disease is known to spread among cattle is through the feeding of infected brain and nerve tissue. Once commonly used as a protein ingredient, ground-up cattle remains were banned from use in cattle feed in 1997.

Cattle remains are still added to food for chicken, pigs and pets; experts believe mad cow disease won't spread to those animals.

Bovine spongiform encephalopathy, or BSE, is the medical name for mad cow disease. In humans, eating infected tissue from cows has been linked to variant Creutzfeldt-Jakob disease, a rare but fatal degenerative disease blamed for the deaths of 150 people in Britain, where there was an outbreak in the 1980s and 1990s.