"The risk to human life is extremely low," Agriculture Secretary Ann Veneman told reporters.
But around the world, more than a dozen countries halted imports of U.S. beef – a $2.6 billion market in 2002 – including the three largest importers, Mexico, Japan and South Korea.
Some stressed that their bans were temporary, until the extent and scope of any infection is confirmed.
Others, like Taiwan and Singapore, emphasized that if the outbreak is confirmed, they would ban U.S. beef for six to seven years, given the long incubation period for the disease, also known as bovine spongiform encephalopathy or BSE.
Canada announced a limited ban, halting imports of some processed beef products but allowing dairy products, live cattle and boneless beef from cattle 30 months of age or younger at slaughter.
Meanwhile, federal and state-level officials worked to trace the Holstein's history before it came to its last home, a large dairy operation near Mabton in southern Washington state, in 2001. USDA chief veterinarian Ron DeHaven said officials have identified two livestock markets in Washington where the animal could have been purchased, but he did not identify them.
But because the brain-wasting disease is usually transmitted through contaminated feed and has an incubation period of four to five years, it is "important to focus on the feed where she was born," DeHaven said.
"Once we have the birth herd, we'll want to know what animals have come into that herd and what animals have left that herd and all the feeding practices for that herd," DeHaven said.
The belief is that the cow got the disease through contaminated feed soon after it was born in 1999, raising the question whether other calves were infected at the same time, reports CBS News Correspondent John Blackstone.
In spite of controls on cattle feed there are loopholes, reports Blackstone, calves are often fed a protein-rich mixture made from dried blood of slaughtered cattle.
When the diseased cow was slaughtered, reports Blackstone, its meat was processed at three different packing plants, one of which was Midway meat in Centralia, Washington.
CBS affiliate KIRO-TV cameras filmed that plant taking in what are called "downer cows," animals that are too sick to walk. The cow believed to be infected with mad cow disease was also a downer cow, but was still sent to slaughter, reports Blackstone.
The human form of the disease, variant Creutzfeldt-Jakob, is thought to be contracted by eating meat from an infected animal, specifically from the brain or spinal cord. Officials stressed that these parts of the sick cow were removed before the rest of the carcass was sent to processing plants.
Agriculture Department officials told a briefing the cow was culled from its herd and slaughtered Dec. 9, after she became paralyzed, apparently as a result of calving. Preliminary tests showed she had mad cow disease.
Tissue samples were sent to Britain's Veterinary Laboratories Agencies, a world leader in mad cow identification, for confirmation. Scientists agreed to forgo their Christmas day off to quickly conduct the tests.
"As soon as it arrives here at the laboratory, our experts have said that they will be able to look at them straight away so we should have a result within a few hours of the initial test," said Steven Edwards, chief executive of Veterinary Laboratories Agencies in Weybridge, west of London.
Consumers were caught by surprise by Tuesday evening's announcement of the first-ever apparent case of mad cow formally known as bovine spongiform encephalopathy in a U.S. cow. Their reaction was mixed.
"I'm content to have a freezer full (of beef)," said Christmas shopper Helen Spinetto of Cambridge, Mass. "But if I had to purchase it again, maybe I'd think twice."
However, the mad cow case reinforced the opinions of Barbara Seaton of the Albany, N.Y., suburb of Colonie. She does not eat beef, and said she would encourage her husband not to. There will "absolutely not" be beef on her holiday table, she said.
On Wall Street, stocks in meatpacking companies and restaurant chains took a hit. Among the losers: McDonald's Corp., Wendy's International, and Tyson Foods, which relies on beef for nearly half its business.
Politically, Democrats jumped on Republicans who removed from a massive agriculture spending bill a ban on processing meat from "downed" animals, those that are ill when they reach the plant. USDA officials have said the Holstein in Washington state was a downer.
Rep. Gary Ackerman has been a leader in supporting the processing prohibition. "This is something that's a potential disaster," the New York Democrat said. "This was so predictable by anybody following the issue."
Investigators were at processing plants in Oregon, where meat from the infected cow had been turned into fresh boneless beef, said a spokesman for the Agriculture Department. Authorities want to know where the meat was sent, although they stressed that the cow's brain and spinal cord, the only parts that are considered able to transmit the disease, did not enter the food supply.
The animal was one of 20 slaughtered Dec. 9 at Vern's Moses Lake Meat Co. in Moses Lake, Wash. All 10,410 pounds of beef from those carcasses have been recalled in "an abundance of caution," Veneman said. The Agriculture Department said there was an extremely low likelihood that the recalled meat could have the mad cow protein.
While the investigation continued, authorities reiterated that America's beef is safe.
"We continue to believe that the risk to human health from this situation is extremely low, and people should continue to feel very confident in the safety of our meat supply," Veneman said.
Washington State University agricultural economist Thomas Wahl told CBS News Correspondent Stephan Kaufman the U.S. food supply is safe, but he admitted that public perception following the first suspected U.S. case of mad cow disease may hurt the beef industry.