WASHINGTON — The Pentagon disclosed Wednesday that it inadvertently shipped possibly live anthrax to at least 51 laboratories across the U.S. and in three foreign countries over the past decade, but it has yet to determine how it happened, who is to blame, why it was not discovered earlier and how much worse the embarrassment will get.

One of the few things Pentagon officials said they were sure of is that public health is not at risk.

“We know of no risk to the general public,” Deputy Defense Secretary Robert Work told a Pentagon news conference. He said the suspect anthrax was shipped in such low concentrations and in such secure packaging that it almost certainly posed no health risk to anyone outside the 51 labs.

The anthrax was supposed to have been killed with gamma rays by Defense Department lab technicians before being shipped for use by commercial labs and government facilities in research and the calibration of biohazard sensors. But for reasons not yet explained, the anthrax apparently remained alive.

To compound the error, follow-up lab tests to verify that the anthrax had been killed before being shipped apparently also failed. One question for the ongoing investigation is whether a sufficiently large sample size of the irradiated anthrax was used in the verification tests, or whether those follow-up tests were even performed.

Officials said the mistakes appear to have begun in 2005 or 2006, although Work said the Pentagon did not become aware of them until alerted May 22 by an unidentified commercial lab in Maryland. That lab reported that supposedly dead anthrax samples it received from an Army laboratory contained live spores.

Navy Cmdr. Franca Jones, director of Pentagon medical programs for chemical and biological defense, told reporters that 31 individuals are receiving antibiotics as a precaution but none are sick. She said 19 of the 51 laboratories that received suspect anthrax have submitted it to the Centers for Disease Control and Prevention for testing, and said that of nine samples fully tested thus far, all nine have proven to contain live anthrax.

“We’ll find more” labs received the suspect anthrax than the 51 notified thus far, Jones predicted, since more than 400 master batches of anthrax at four Defense Department laboratories that are responsible for shipping it to commercial laboratories have yet to be tested for live anthrax spores.

Of four batches fully tested thus far — all at the Army’s Dugway Proving Ground in Utah — all four have been determined to contain live anthrax, even though the material had undergone irradiation in accordance with a well-established but apparently flawed protocol for killing the anthrax.

Jones said the samples from those master batches were shipped as long ago as 2006. She did not provide a full timeline for the shipments, although some were received in recent months.

The three other Defense Department labs that are authorized to perform similar functions with anthrax are in Maryland, according to a Pentagon spokesman, Col. Steve Warren. He said the Pentagon is legally prohibited from disclosing the names of the dozens of commercial laboratories in 17 states and the District of Columbia that received suspect anthrax. They are in California, Delaware, Maryland, Massachusetts, New Jersey, New York, Tennessee, Texas, Utah, Virginia, Washington, Wisconsin, Arizona, Florida, Illinois, Ohio, North Carolina and the District of Columbia.

The scope of the problem has grown almost daily since the Pentagon first acknowledged it publicly May 27. On that date the Pentagon reported that labs in nine states were affected. A short time later it said the suspect anthrax also had been received at a U.S. military facility at Osan Air Base in South Korea. By the end of the week the number had grown to 25 labs in 11 states plus Australia and Canada. At that point Work, the deputy defense secretary, ordered a comprehensive review of laboratory procedures associated with killing live anthrax.

Work said the Pentagon will take steps to hold people accountable for the lapses once the CDC has completed its investigation into what happened at Dugway.