A US military facility in Utah mistakenly sent live anthrax bacteria to private laboratories in nine US states and a US military base in South Korea, the Pentagon has said.
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 four face "minimal" risk, said Jason McDonald, a spokesman for the US Centers for Disease Control and Prevention (CDC).
The four were "doing procedures that sent the agent into the air," he said.
When anthrax becomes airborne, it can cause a deadly illness called inhalation anthrax.
That is what happened in 2001, when anthrax sent through the US post to government and media targets killed five people.
The anthrax sent from the Utah military lab was meant to be shipped in an inactive state as part of efforts to develop a field-based test to identify biological threats, the Pentagon said.
"Out of an abundance of caution, (the Defense Department) has stopped the shipment of this material from its labs pending completion of the investigation," said Pentagon spokesman Colonel Steve Warren.
The CDC said it had launched an investigation.
Previous safety lapses at government labs
The incident comes 11 months after CDC, one of the government's top civilian labs, similarly mishandled anthrax.
Researchers at a lab designed to handle extremely dangerous pathogens sent what they believed were killed samples of anthrax to another CDC lab, one with fewer safeguards and therefore not authorised to work with live anthrax.
Scores of CDC employees were potentially exposed to the live anthrax, but none became ill.
That incident and a similar one last spring, in which CDC scientists shipped what they thought was a benign form of bird flu but which was actually a highly virulent strain, led US politicians to fault a "dangerous pattern" of safety lapses at government labs.
In the latest case, anthrax was sent from the Army's Dugway Proving Ground to laboratories in Maryland, Texas, Wisconsin, Delaware, New Jersey, Tennessee, New York, California and Virginia, a defence official said.
The four civilians receiving post-exposure prophylaxis are in Delaware, Texas, and Wisconsin.
The sample sent to South Korea was subsequently destroyed, the Pentagon said.
