It will take days to account for all the erroneously mailed batches, the military said at a news conference Wednesday at the Pentagon.
The scope of the apparent error has mushroomed and grown to include at least two labs in Manitoba and Alberta that received potentially contaminated experiment kits.
"We will find more labs, we will find more lots," said Cmdr. Franca Jones, director of the military's chemical and biological defence programs.
"(But) I don't believe we'll find that there are 1,000 labs out there that have received this material."
In the week since the military acknowledged the error, the number of cases has surged. Just last week, the military had identified 24 labs in 11 states as having received potentially faulty test kits. By Wednesday, that had grown to 51 laboratories in 17 states as well as Canada, Australia and South Korea.
The good news, officials insisted, was that there's no widespread threat: "We believe that the risk is zero for the general public," said defence undersecretary Frank Kendall.
During Wednesday's news conference, a demonstration showed how one-millilitre vials of liquid anthrax were sealed in plastic bags; then swathed in an absorbent wrapper; placed in a polycarbonate container; packaged in dry ice; and finally put into a box before being shipped.
Any threat would be to lab workers opening those packages and working with the contents, they said. Even then, the anthrax samples were in liquid form — not powder, which limits the risk of airborne exposure which is far more lethal than skin contact.
The military is now conducting different investigations.
One is to determine what happened, and who's responsible. Pentagon officials said there's no indication of criminal intent so far. But for whatever reason, something went wrong with several batches: they were supposed to be blasted with gamma rays, to kill the bacteria.
Not only were they not properly irradiated — they were then mislabelled and finally distributed. A preliminary report is expected within 30 days.
A more urgent search is also underway.
U.S. officials have begun testing 400 batches of lab anthrax distributed over the last decade. At least four batches were contaminated with live anthrax. There could be many more. Among the remaining 396, none has yet cleared the 10-day period required for a conclusive, negative test.
Nor can they confirm how many individual lab units the average batch contained — and wouldn't say whether the number of affected samples might be in the thousands.
At-risk batches were linked to three kits sent to Canada: one to Winnipeg's National Microbiology Laboratory and two that were sent in August 2007 to a research facility in Suffield, Alta.
The kits are used to test a lab's proficiency at detecting pathogens.
There are no reports of illness linked to the shipments in Canada. The Winnipeg lab said in an email that its kit has been moved to a higher security area in the centre.
Four people at labs in Delaware, Texas and Wisconsin have been told to get antibiotics as a precaution, but were not ill. U.S. officials said 22 people at a base in South Korea were being treated for possible exposure.