More than 25 people are receiving preventative treatment after the US military "inadvertently" shipped live Anthrax samples to facilities in as many as nine states and a US military base in South Korea.
Workers at a laboratory in Maryland were the first to report that a live sample had been received from Dugway Proving Ground, a US army base in Utah.
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A Pentagon spokesman said there was "no known risk to the general public" and that there were "no suspected or confirmed cases of Anthrax infection in potentially exposed lab workers".
Anthrax is spread by contact with spores but is not contagious among humans. It can manifest in the lungs, skin or intestine, causing symptoms such as an upset stomach, headaches, breathing difficulties and even death.
The army base in Utah was said to be developing a test to identify biological threats and had believed the Anthrax samples were dead or deactivated when they were shipped out for research purposes.
Four Defense Department workers and 22 people at the Osan Air Base in South Korea are undergoing post-exposure treatment, including examinations, antibiotics and vaccinations. However, none have shown any signs of exposure.
The Centres for Disease Control and Prevention (CDC) is also investigating the incident.
Richard Sisk at Military.com says Dugway Proving Ground, a "vast military facility about the size of Rhode Island", has long been the military's main centre for testing chemical and biological weapons. In 2011, Dugway was locked down for 13 hours after a one-millilitre vial of VX nerve agent went missing. It was eventually located and nobody was hurt, but it was described as a "serious mishandling" of the chemical.