The government closed its investigation into the nation's first domestic case of mad cow disease (search) Tuesday, saying it could not pin down how a Texas cow was infected with the brain-wasting ailment.

Officials continue to believe the 12-year-old Brahma cross cow (search) ate contaminated feed before the United States banned ground-up cattle remains in cattle feed.

The only way the disease is known to spread is through eating brain and other nerve tissue from infected cows.

"The investigation did not identify a specific feed source as the likely cause of this animal's infection," said Steve Sundlof, director of the Food and Drug Administration's (search) Center for Veterinary Medicine.

Sundlof said the most likely culprit was tainted feed eaten before the 1997 ban.

The Texas-born cow tested positive in June.

The feed ban has loopholes allowing cattle to be fed poultry litter, cattle blood and restaurant leftovers, all potential pathways for mad cow disease. FDA officials promised last year to close the loopholes; Sundlof said the agency will act within the next two months.

The Agriculture Department and FDA said the investigation indicated there was no danger to human or animal health. The investigation also found:

— 21 types of feed or feed supplements were used since 1990 on the cow's farm, which the government has not identified. Nine feed mills and three retail feed stores supplied the feed ingredients.

— 147 herd mates and offspring were presumed to have been slaughtered for food, feed or other use, and 21 could not be traced. USDA located, killed and tested 67 animals, all of which tested negative for mad cow disease. In all, the government traced 413 animals in its investigation.

Also Tuesday, officials agreed to let the industry run a nationwide system of tracking the movements of cows, pigs and chickens from birth to the dinner table.

The Agriculture Department had vowed to hustle the system into place after discovering the nation's first case of mad cow disease in December 2003 in an imported cow believed to have been infected in Canada, where it was born.

The goal is to have a mandatory reporting and registration system that would allow an animal to be traced within 48 hours after a disease is discovered.

"It simply makes good sense for producers to design and maintain that piece of the system," Agriculture Secretary Mike Johanns said. Many cattle ranchers are wary of a government-run system and want their records kept confidential.

The move was applauded by the dominant cattle ranchers' group, the National Cattlemen's Beef Association, which is creating its own tracking system and hopes the department will rely on it.

"Protection of producers' rights and confidentiality is a top priority, and the industry is best equipped to do this," said Mike John, a Missouri cattle producer and president-elect of the group.

Consumer groups and the National Farmers Union criticized the move. The system "should not be a revenue source for entities seeking to make a profit," said farmers' union president Dave Frederickson.

An industry-run system is unlikely to reassure consumers in the U.S. and abroad that their beef is safe, said Carol Tucker Foreman, director of food policy at Consumer Federation of America.

The medical name for mad cow disease is bovine spongiform encephalopathy, or BSE. In people, eating meat products contaminated with BSE has been linked to about 150 deaths from a rare degenerative disease called variant Creutzfeldt-Jakob disease.