The US Department of Agriculture said on Saturday it believed the cow diagnosed with bovine spongiform encephalopathy (BSE) was imported from the western Canadian province of Alberta in August 2001.
But the chief Canadian veterinarian said it was too early to reach firm conclusions.
"As yet, there is no definitive evidence that confirms that the BSE-infected cow originated in Canada," said Dr Brian Evans.
If the cow did come from Canada, it would deal another heavy blow to the country's cattle industry, still trying to recover from a single case of mad cow disease discovered in Alberta in May of this year.
Evans said the US needed to carry out a full scientific probe into the case which would then have to be submitted to an international panel of experts for peer review.
This week's discovery of the first US case of mad cow disease in a six and a half year-old Holstein dairy cow has halted most US exports of beef, sent food company stocks tumbling and shaken consumer confidence.
Second biggest buyer of US beef,
South Korea, has halted imports
Some two dozen countries including Japan have banned US beef, seriously threatening the $27 billion US cattle industry.
A USDA team of trade experts has left for Japan, the top US buyer, and will begin talks on Monday about how to address that nation's concerns and resume beef shipments.
Scientists believe mad cow disease is spread to other cattle by the consumption of feed contaminated with diseased brain or spinal column material.
An outbreak of BSE forced the slaughter of millions of cattle in Europe in the 1990s. At least 137 people, mostly in Britain, died of a human form of the disease, known as variant Creutzfeldt-Jakob disease.