The Pentagon disclosed on Wednesday that it inadvertently shipped possibly live anthrax to at least 51 laboratories across the US and in three other 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 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.