The US military has said that it discovered even more suspected shipments of live anthrax to research laboratories than previously thought, both within the country and abroad, prompting a thorough review of its practises.
In a statement issued on Friday evening, the Pentagon said 24 laboratories in 11 states and two foreign countries - South Korea and Australia - are believed to have received suspect anthrax samples.
It had previously only identified an accidental foreign shipment to a US airbase south of Seoul, the South Korean capital.
"There is no known risk to the general public and an extremely low risk to lab workers," the Pentagon said in a statement.
Still, in a sign the Pentagon was still coming to grips with the extent of the problem, it advised all laboratories for now to stop working with any "inactive" samples sent from the defence department.
To date, the US has acknowledged that four US civilians have begun taking preventive measures that usually include the anthrax vaccine, antibiotics or both.
Twenty-two people at the base in South Korea were also given precautionary medical measures although none of them has shown signs of exposure, officials said.
The suspected live samples identified so far all appear to trace back to a US army base in Utah, the Dugway Proving Ground, one of the military labs responsible for inactivation and shipping of biological material.
The US military disclosed earlier this week that suspected live samples sourced to Dugway were traced going to nine US states and a US airbase in South Korea. A US official said those shipments took place between March 2014 to April 2015 before being discovered this month.
On Friday, US officials said the suspect sample sent to Australia came from a 2008 batch from Dugway.
Kristen Nordlund, spokesperson for the Centers for Disease Control and Prevention, said the agency is testing to see which anthrax samples were live. The results are coming in slowly, she said, and the first full set of findings is not expected until next week.
Anthrax is a potentially deadly infectious disease caused by exposure to the bacterium Bacillus anthracis. The bacteria most commonly affect hoofed animals such as goats, but people can also become infected.
Infection can occur through a cut in the skin, breathing in anthrax spores or eating tainted meat.
The normal incubation period for anthrax can take up to five to seven days, though there are documented cases of the illness occurring about 60 days after exposure.