Ron DeHaven, the USDA\'s chief veterinarian, told reporters the animal was likely one of a herd of 74 dairy cows imported into Idaho from Alberta, Canada, in August 2001.
But in Ottawa, chief Canadian veterinarian Dr Brian Evans told a news conference: \"It would be premature to draw such a conclusion at this time ... As yet, there is no definitive evidence that confirms that the BSE -infected cow originated in Canada.\"
In May this year, Canadian officials reported the discovery of a single case of mad cow disease in a black Angus cow in Alberta, but the USDA said it was too early to speculate on whether the two cases were related.
This week\'s discovery of the first U.S. case of mad cow disease in a 6-1/2 year-old Holstein dairy cow has halted most U.S. exports of beef, sent food company stocks tumbling and shaken consumer confidence.
Some two dozen countries including Japan have banned U.S. beef, seriously threatening the $27 billion U.S. cattle industry. A USDA team of trade experts has left for Japan, the top U.S. buyer, and will begin talks on Monday about how to address that nation\'s concerns and resume beef shipments.
\"The positive animal likely entered the United States as part of a group of 74 dairy cattle that were imported through the border crossing at Eastport (in Idaho),\" DeHaven said.
He said the imported cows were sent to a dairy farm in Mattawa, Washington and the infected cow was later sold to another dairy operation in Mabton, Washington. The USDA is still unsure where the cow was born.
The USDA, which has already quarantined two Washington herds, is investigating where the other 73 dairy cows went.
\"We feel confident that we are going to be able to determine the whereabouts of most, if not all, of these animals within the next several days,\" DeHaven said.
Evans said Canadian and U.S. authorities were cooperating closely to track the origins of the cow, but the United States needed to carry out a full scientific probe which could be submitted to international experts for peer review.
\"It is imperative that all the evidence be weighed and verified before anyone jumps to any predetermination,\" he said. \"There is still much investigative work to be done... we have concerns that we feel need to be worked through and verified.\"
The U.S. National Cattlemen\'s Beef Association said its trading partners should allow U.S. beef shipments to resume if the infected cow came from Canada.
The USDA said meat linked to the infected cow was sold in Washington, Oregon, California and Nevada.
\"It\'s too early for us to know how much of it has been sold and ... whether it has in fact been consumed,\" said Kenneth Petersen of USDA\'s Food Safety and Inspection Service.
Safeway Inc., Fred Meyer and Albertsons Inc. have asked customers to return certain cases of beef patties and other products that originated at Vern\'s of Moses Lake Meats, which slaughtered the infected cow. The plant this week recalled more than 10,000 pounds of raw ground beef.
Bush administration officials repeated assurances that the beef supply was safe for consumers.
Scientists believe mad cow disease is spread to other cattle by the consumption of feed contaminated with diseased brain or spinal column material. The Food and Drug Administration said it had located all \"high risk\" material from the infected cow.
A USDA spokeswoman said the department would reassess its proposal to reopen the border to young Canadian cattle due to the new findings.
An outbreak of mad cow disease, or bovine spongiform encephalopathy (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 . (Additional reporting by David Ljunggrenn in Ottawa)