The Pentagon has said that a suspected live sample of anthrax was sent to a US air base in South Korea - in addition to other samples mistakenly sent to nine US states.
But the Pentagon stressed no personnel have shown signs of possible exposure to the virus. It said there was no known suspected infection or risk to the public.
However, four US civilians have been started on preventive measures called post-exposure prophylaxis, which usually includes the anthrax vaccine, antibiotics or both.
The US Centres for Disease Control and Prevention (CDC) said Wednesday it is now investigating what the Pentagon called an inadvertent shipment of live anthrax spores to government and commercial laboratories in as many as nine states, as well as one overseas, that expected to receive dead spores.
A Pentagon spokesman, Colonel Steve Warren, said the suspected live anthrax samples were shipped from Dugway Proving Ground, an Army facility in Utah, using a commercial delivery service.
Later he said an anthrax sample from the same batch at Dugway also was sent to a US military laboratory at Osan air base in South Korea - but the sample was destroyed.
All samples involved in the investigation will be securely transferred to the CDC or other laboratories for further testing, reports the Huffington Post.