The government was trying to reassure the public about the safety of the U.S. food supply even as it confronted a wide ban on U.S. beef by countries that account for 90 percent of American beef exports.
The recall of more than 10,000 pounds of meat from the cow and others slaughtered Dec. 9 at the same Washington company also was continuing.
White House spokesman Scott McClellan said President Bush will continue to eat beef, adding that the president's focus "is on the public health aspect of this."
The quarantine, which now includes herds at two Washington farms, was imposed even though officials said transmission of the disease from mother to calf is considered unlikely.
One calf is at the same dairy near Mabton, Wash., that was the final home of the diseased Holstein cow, said Dr. Ronald DeHaven, the Agriculture Department's chief veterinarian. The other calf is at a bull calf feeding operation in Sunnyside, Wash., DeHaven said.
A third calf died shortly after birth in 2001, he said.
"The reason for concern with these calves is that even though it is an unlikely means of spreading the disease, there is the potential that the infected cow could pass the disease onto its calves," he said. No decision has been made on destroying the herds, he said.
The emphasis of the widening investigation is on finding the birth herd of the cow, since it likely was infected several years ago from eating contaminated feed, DeHaven said. Scientists say the incubation period for the disease in cattle is four or five years.
DeHaven called the investigation "a tangled web of possibilities," saying the cow's path could lead to other states or Canada.
Tracing the source of the infected cow could take days or weeks, he noted, adding that Canadian officials worked for several weeks to locate the birth herd of a cow with brain-wasting disease earlier this year.
"If we're lucky, we could know something in a matter of a day or two," DeHaven said. However, he added, that it is possible that officials may never definitively identify the herd or the source of contaminated feed.
But food safety advocates say the government by now should know where those animals are, reports CBS News Correspondent Wyatt Andrews.
"The gaps in this system for protecting the public are huge," said Caroline Smith DeWaal, food safety director for the Center for Science in the Public Interest.
She and other experts believe the government has taken too long adopting so-called animal trace-back technology, which the U.S. says it is working on, and which could quickly locate and identify all other cows that may be a mad cow threat.
"If we can't identify the cattle that potentially were infected at the same time this one was the USDA really cannot protect the public," Smith DeWaal said.
The former inspector general of the Agriculture Department, Roger Viadero, says the government could know where every suspect cow came from and where every bit was sold, but never pursued the system.
"In ten minutes they should have been able to tell where the animal came from and where it went," said Viadero.
Confirmation of the first case of mad-cow disease in the United States came Thursday from the Veterinary Laboratories Agency in Weybridge, England. British researchers agreed with the reading of U.S. tests on the stricken cow that showed it had the brain-wasting disease.
BSE is caused by a misshapen protein — a prion — that eats holes in a cow's brain. A total of 153 people worldwide have been reported to have contracted the human form of the illness, according to the federal Centers for Disease Control and Prevention.
Government officials insisted there was no threat to the food supply because the cow's brain, the spinal cord and the lower part of the small intestine — where scientists say the disease is found — were removed before it was sent on for processing.
Humans can contract a fatal variant of mad cow disease by eating infected beef products, but experts say muscle cuts of beef — including steaks and roasts — are safe.
Still, many countries banned American beef products after Agriculture Secretary Ann Veneman first announced a probable case of mad cow disease, officially known as bovine spongiform encephalopathy, on Tuesday. USDA chief economist Keith Collins said that 16 countries that account for 90 percent of U.S. beef exports have barred American beef products.
A U.S. delegation will leave Saturday for Japan and possibly other Asian countries in an effort to minimize the impact on American beef producers, DeHaven said. Japan, which has halted U.S. beef imports, bought $1.03 billion worth of U.S. beef in 2002, about a third of exports, Collins said.
Japan suffered an outbreak of mad cow disease in 2001. More recently, it found a 23-month-old cow sick with the illness in October, that prompted scientists to say testing programs need to be changed.
Currently, the United States tests for the disease in animals 30 months old and older, as well as those that exhibit central nervous system disorders or are unable to stand or walk on their own.
Agriculture officials have discussed expanding their testing program, which critics say is insufficient. One proposal would be to test all cows that get sick and die on a farm, even if mad cow is not suspected.