The United States authorities are trying to establish the origin of the feed that may have infected a cow with BSE, or mad cow disease.
The cow was born before bans on cattle remains in feed
The Food and Drug Administration said the cow was born before a ban on feeding cattle remains to livestock.
"We're... trying to determine where feed may have come from," said Stephen Sundlof from the administration.
There is now a recall on beef following discovery of the case - which is the first in the US.
Officials had said the meat from the infected cow, slaughtered on 9 December in Washington State, had only gone to Oregon, California, Nevada and Washington.
But now they are saying the beef could have gone to outlets in Alaska, Hawaii, Montana, Idaho and the territory of Guam.
Dr Ron DeHaven from the US Agriculture Department said that DNA tests now under way would provide
more information on the origin of the cow that was infected.
They also are looking at records of the herd that included the infected cow.
She had three calves after she entered the US, officials said.
One died, while another remains in a
herd in the state of Washington.
A third calf, a male, is being
held in isolation with other animals, officials said.
It is unlikely that the disease has been transmitted to the calves, said Dr DeHaven.
BOVINE SPONGIFORM ENCEPHALOPATHY
First surfaced mid-1980s
Can pass to humans through infected beef products
Human form of disease called vCJD
vCJD has killed 137 people, mainly in the UK
"Even with the finding of this single cow, the US remains at very low risk," he said.
US officials say American beef is still safe because the infected cow's brain and spinal cord - which is the only part normally tainted by the disease - was removed before the meat was processed.
As investigations continue into how the cow became infected with "mad cow" disease, or BSE, officials in Canada have urged the US not to "rush to judgment".
Dr DeHaven said the dairy cow found to have BSE was believed to have been one of 74 imported from Canada into Idaho in 2001.
He said it was not yet known where the other 73 animals imported from Canada were, adding that they may or may not be infected.
SELECTED COUNTRIES THAT HAVE BANNED US BEEF
Canada - partial ban
United Arab Emirates
*Top three importers
But his Canadian counterpart, Dr Brian Evans, said the details on the cow's records in the US did not match those kept in Canada.
"As yet, there is no definitive evidence that confirms that the BSE-infected cow originated in Canada," Dr Evans said.
Canada reported its own first case of mad cow disease - or bovine spongiform encephalopathy - in Alberta last May. It is not known if the US and Canadian cases are connected.
Correspondents say the Canadian beef industry would suffer again if the Canadian link was confirmed - just as it was beginning to recover after exports fell in the wake of the May case.
In the US, experts have predicted this week's news of the BSE case will cost the US cattle industry billions of dollars. At least 26 countries have now banned imports of American beef because of BSE fears.