Government investigators are tracing the history of an Alabama cow that has become the nation's third case of mad cow disease.

The Agriculture Department confirmed the infection Monday. Unable to walk, the cow was killed last week by a local veterinarian and buried on the farm.

"This animal did not enter the human food or animal feed chains," the department's chief veterinarian, John Clifford, told reporters during a Monday conference call.

Word came as the Bush administration sought to reassure Japan, South Korea and other trading partners that U.S. beef is safe. The United States is still working to recover some markets that were cut off after the first U.S. case of mad cow disease in 2003.

"If the infected cow turns out to have been born before April 1998, when the South Korean government banned meat and bone meal, it will not influence the South Korea-U.S. beef import agreement in January," the South Korean Ministry of Agriculture and Forestry said in a statement issued Tuesday in Seoul.

Under the January agreement, South Korea planned to resume limited imports of U.S. beef by the end of this month.

The animal was a Santa Gertrudis breed, a red-colored beef cow that thrives in hotter weather in the southern United States.

Federal and state investigators are working to determine where the cow was born and raised and find any herdmates or offspring.

They also want to pinpoint its age. Based on the animal's teeth, the local vet reported that the cow was older, "quite possibly upwards of 10 years of age," Clifford said. But he said it can be hard to tell the age of older cows from their teeth.

"It's an estimate," he said. "We may not be able to determine the exact age. But we're going to do everything we can to trace this animal back to the herd of origin and determine its age."

The age of the cow is important because the U.S. put safeguards in place nine years ago to prevent the disease from spreading. The U.S. banned ground-up cattle remains from being added to cattle feed in 1997. Eating contaminated feed is the only way cattle are known to contract the disease.

Older animals are more likely to have been exposed to contaminated feed circulating before the 1997 feed ban.

In Canada, which enacted a similar feed ban in 1997, the most recent case of mad cow disease was an animal born after the feed ban, raising questions about enforcement. That case was confirmed in January in Alberta.

In Alabama, state authorities said the farm was under an informal quarantine but would not say where it was. The cow had spent less than a year there before it died, officials said.

"We will not release this information at this time until we complete our investigation, and that could take a few days," state agriculture commissioner Ron Sparks said.

Sparks said there were no suspect animals on the farm.

The first U.S. case of mad cow disease appeared in December 2003 in a Canadian-born cow in Washington state. The disease was found again last June in a cow that was born and raised in Texas.

Japan, once the top international buyer of American beef, closed its market after the first U.S. case and had only recently lifted its ban. However, Japan halted U.S. beef shipments in January after finding veal cuts with backbone -- cuts that are eaten in the U.S. but not in Asia.

"We would not anticipate that this would impact our ongoing negotiations," Clifford said. "Our product is safe. We've got a number of interlocking safeguards. And Japan themselves have had 20-plus cases of BSE."

Mad cow disease is the common name for bovine spongiform encephalopathy, or BSE. In humans, eating meat products contaminated with BSE has been linked to more than 150 deaths, mostly in Britain, from variant Creutzfeldt-Jakob Disease, a rare and fatal nerve disease.