Viral fragments of bird flu have been identified in samples of milk taken from grocery store shelves in the United States, a finding that does not necessarily suggest a threat to human health but indicates the avian flu virus is more widespread among dairy herds than previously thought, according to two public health officials and a public health expert who was briefed on the issue.

The Food and Drug Administration said Tuesday that it had been testing milk samples throughout the dairy production process and confirmed the detection of viral particles “in some of the samples,” but it declined to provide details.

The presence of genetic fragments of the virus in milk is not unexpected. Pasteurization typically works to inactivate pathogens, said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health. It generally does not remove genetic material, Nuzzo said, but typically renders pathogens unable to cause harm to people.

The greater concern, however, “is that it’s showing up in a lot more samples, meaning the infection is more widespread in dairy herds than we thought,” said one public health official, who spoke on the condition of anonymity to share information not yet made public.

In a four-page statement, the FDA said some of the samples collected have “indicated the presence” of the bird flu virus based on testing that detects viral particles but does not distinguish whether they are active or dead. The finding of virus “does not mean that the sample contains an intact, infectious pathogen,” the agency’s statement said.

Additional laboratory testing is underway to grow the virus in cells and in fertilized eggs, the latter being the “gold standard” for sensitive detection of active, infectious virus, the FDA said. “Importantly, additional testing is required to determine whether intact pathogen is still present and if it remains infectious, which determines whether there is any risk of illness associated with consuming the product,” the FDA statement said.

FDA officials said results are expected in the next few days to weeks.

“To date, we have seen nothing that would change our assessment that the commercial milk supply is safe,” the agency said in its statement.

Officials and experts did not have additional details about the number of milk samples that were positive for particles of bird flu or where the samples originated.

Although this strain of avian flu has been circulating for more than 20 years, its leap into cows is of substantial concern, surprising even longtime observers of the virus. More than two dozen livestock herds in at least eight states have been infected with avian flu since March 25, prompting investigations by federal and state officials.

For weeks, key federal agencies have expressed confidence in the safety of the commercial milk supply, including pasteurized products sold at grocery stores. The FDA has highlighted data showing pasteurization inactivates other viruses and pointed to studies showing that the pasteurization process for eggs — which occurs at a lower temperature than what is used for milk — deactivates the highly pathogenic avian influenza.

The International Dairy Foods Association, which represents the nation’s dairy manufacturing and marketing industry, said that viral fragments are “nothing more than evidence that the virus is dead.”

“Milk and milk products produced and processed in the United States are among the safest in the world,” spokesman Matt Herrick wrote in an email, adding that “viral fragments are simply indicative of pasteurization doing its job effectively and protecting our commercial milk supply.”

In recent weeks, multiple experts expressed confidence that the pasteurization process ensures there is no threat to the safety of the nation’s milk supply but said the federal government should still perform tests to confirm that is the case.

Flu is a “fairly wimpy virus,” meaning it is “fairly readily inactivated,” said Richard J. Webby, a virologist at St Jude Children’s Research Hospital. “But that’s something that has to be tested.”

One case of avian flu has been reported in a Texas farmworker in recent weeks, only the second human case ever of bird flu in the United States.

So far, the virus has not acquired the ability to spread efficiently in people.

But as it is able to jump from animal to animal, prospects increase for it mutating to cause sustained person-to-person transmission — a development that could fuel a pandemic.

State health officials have tested 23 people with flu-like symptoms, but only the dairy worker in Texas has tested positive during the current outbreak. Ongoing surveillance of emergency department visits and flu tests in regions where bird flu has been detected has not flagged unusual trends in flu-like illnesses, or eye inflammation, the only symptom experienced by the dairy worker, according to officials at the Centers for Disease Control and Prevention, who say the risk to the general public of bird flu remains low.

The lack of more human cases is a good sign, health officials say.

The key to containing the outbreak resides in livestock herds. Testing of cows is voluntary. U.S. Department of Agriculture protocols restrict testing to cows with specific symptoms and limits the number of tests per farm.