The image released by US Department of Defense shows Bacillus anthrax vegetative cells and spores. (AFP)
The US military shipped live anthrax to an American base in Japan a decade ago, bringing to five the number of countries that were inadvertently sent the lethal bacteria, officials said Friday.
The sample was sent in 2005 to Camp Zama southwest of Tokyo and was destroyed in 2009, Pentagon spokesman Colonel Steve Warren said.
"There currently is no anthrax -- activated or inactivated -- in Japan at this time," Warren said.
South Korea, Canada, Australia and Britain have also received active anthrax bacteria, the Pentagon said, as well as 19 American states and the US capital Washington.
In all, 69 labs have received live anthrax samples since 2005.
Warren said investigators were looking into "some sort of mechanical problem (or) some sort of problem with the underlying science behind deactivation."
"Of course, human error is a possibility," he added.
No one has fallen sick from handling the samples, and the concentration of active bacteria is extremely low.
Senior defense officials admitted last week the problem of mistaken shipments was worse than initially reported and that the extent of the problem remained unclear.
But officials have insisted there is no threat to public health and that the small concentrations of live anthrax had been shipped in several layers of air-tight packaging.
The military has tested a total of 91 "lots" of what were believed to have been inactivated anthrax. Of those batches, 11 have so far tested positive for containing activated anthrax.
So far, all of the active bacteria has originated from Dugway Proving Ground, a US Army lab in Utah.