Federal regulators have discovered fragments of the bird flu virus in about 20 percent of retail milk samples tested in a nationally representative study, the Food and Drug Administration said in an online update Thursday.

Samples from parts of the country known to have dairy herds infected with the virus were more likely to test positive, the agency said. Regulators said there is no evidence that this milk poses a danger to consumers or that there are live viruses in the milk on store shelves, an assessment that public health experts agree with.

But finding traces of the virus in such a high proportion of samples from around the country is the strongest sign yet that the bird flu outbreak in dairy cows is larger than the official count of 33 infected herds in eight states.

“This suggests that there is a lot of this virus out there,” said Richard Webby, a virologist and influenza expert at St. Jude Children’s Research Hospital.

Dr Webby said he believed it was still possible to eradicate the virus, known as H5N1, from the country’s dairy farms. But it will be difficult to design effective control measures without knowing the extent of the outbreak, he said.

The findings also raise questions about how the virus has evaded detection and where else it might be silently spreading. Some scientists have criticized the federal testing strategy as too limited to reveal the true extent of viral spread.

Until Wednesday, when the Department of Agriculture announced mandatory testing of dairy cows crossing state lines, cow testing had been voluntary and focused primarily on cows with obvious symptoms.

As of Wednesday, only 23 people had been tested for the virus, while 44 people were being monitored after exposure, according to the Centers for Disease Control and Prevention.

A widespread outbreak in cows would pose a greater risk to farm workers, the dairy industry and general public health. Sustained spread among cows would give the virus more opportunities to acquire mutations that make it more transmissible among humans.

The FDA did not provide details Thursday about the quantity or origin of the samples.

“Not only do you want to go to places where you know there is activity and cows, but you want to go to places where at least no bird flu has been reported,” Dr. Webby said.

Experts believe that the pasteurization process, in which milk is briefly heated, should inactivate this bird flu virus, known as H5N1.

“And when the virus is destroyed, genetic material is released,” said Samuel Alcaine, a microbiologist and food scientist at Cornell University. The genetic fragments that remain are not capable of causing infection.

“It’s not surprising” to find them in milk, he added. “That doesn’t mean milk isn’t safe.”

Federal officials are still conducting the time-consuming tests to determine whether any viable virus remains in the milk after pasteurization. Scientists have said that prospect is highly unlikely.

Dr. Jeanne Marrazzo, director of the National Institute of Allergy and Infectious Diseases, said at a news conference Wednesday that some federally sponsored researchers had tested live viruses in retail milk but found none, a sign that pasteurization had killed the virus. before milk hit supermarket shelves.

Dr. Marrazzo cautioned that while the results were a small sample, the findings were “good news.”

“To really understand the scope here, we have to wait for the FDA’s efforts,” he said.

Finding traces of the virus in 20 percent of commercial milk samples does not mean that 20 percent of the country’s dairy herds are infected, experts warned. “It’s too early to try to do that kind of superficial calculation,” Dr. Alcaine said.

Milk from several farms is generally pooled. If the virus appears in many milk samples drawn from a group, it could mean that many cows are infected, or that a smaller number of infected cows are shedding large amounts of virus, Dr. Alcaine said.

Even in the latter case, however, a 20 percent positivity rate would suggest there are more than 33 infected herds, he noted.

At Wednesday’s news conference, Dr. Donald A. Prater, acting director of the FDA’s Center for Food Safety and Applied Nutrition, noted the novelty of the research effort. Studies on the effects of pasteurization on the bird flu virus in milk have never been completed, he said.

Regulators were examining milk at various points in the commercial supply chain, he added, including milk on supermarket shelves, as well as studying potential differences between dairy products, such as those between whole milk and cream. said Dr. Prater.