However, British authorities said the samples, from 2007, were no longer a threat.
"We do not believe there is any continuing health risk to staff or to the public," the UK's Health and Safety Executive said in a statement, noting that the company at issue -- which is not being named -- has been in touch with the Defense Department about the matter.
South Korea, Australia and Canada have already been named as recipients of questionable anthrax samples, along with 19 states and the District of Columbia.
Deputy Secretary of Defense Robert Work said last week that he expects the numbers of labs suspected of receiving live anthrax to go up as the Pentagon continues its investigation into the shipments, some of them made via FedEx. When put in the mail, the samples were not believed to contain live samples of the disease.
There have been "no suspected or confirmed cases of anthrax infections" as a result of the shipments, Work said, explaining that the samples that were sent out had very low concentrations of the deadly disease, which could not infect the "average healthy individual."
"We know of no risk to the general public from these samples," Work emphasized.
The four Defense Department laboratories that stockpile anthrax samples for research will test all previously "inactivated" samples to ensure that the anthrax is in fact dead. The department is testing over 400 batches, with live anthrax found so far in four of those batches.
"That is why the numbers may rise," Work said, adding that it takes 10 days to test anthrax samples.
The Pentagon will investigate why the anthrax samples were not properly killed before they were shipped and what protocols and procedures failed in the process.
The press conference was the first public accounting of the investigation into the shipment of live anthrax samples, which was initially reported last week.
The Pentagon Force Protection Agency, the Pentagon's police force, is one of the agencies that received questionable U.S. Army shipments of anthrax. That shipment now must be tested to see if it has live rather than dead pathogen.
The Pentagon police received a shipment of what was supposed to be dead anthrax agent from one of three original lots, all of which are now shown to contain live, rather than dead, anthrax.