Investigators say the Alabama cow that was recently diagnosed with mad cow disease may have been infected before the U.S. took steps to protect cattle feed from it.
New reports show that the infected cow was about 10 years old.
The government started taking steps to protect cattle feed from the disease about nine years ago.
Documentation on an Alabama cow infected with mad cow disease has only been tracked to its most recent seller and agriculture officials say they are not sure whether they would ever figure out its origins.
Alabama Agriculture Commissioner Ron Sparks said investigators are interviewing the farmer and possible sellers, but that it "might not be possible" to pinpoint where the cow came from.
The cow, a red crossbreed, had at least two offspring. The youngest, a 6-week-old calf, was transferred to a U.S. Department of Agriculture lab in Ames, Iowa, for observation.
A second calf, which was about 14-months old, died "quite sometime ago" of natural causes, according Kenneth Angel, the USDA area epidemiology officer for Alabama. He declined to give the location and specific time of death for the calf.
Sparks declined to reveal the farm at which the cow had spent the last year, saying he didn't want to hurt the farm's business or discourage other farmers from reporting "downer" animals, or those unable to walk, which is considered an indicator of mad cow.