As many as 75 workers at the US Centers for Disease Control and Prevention (CDC) may have been exposed to anthrax bacteria and are receiving treatment.
The US health agency said researchers in a high-level biosecurity laboratory failed to follow proper procedures and did not inactivate the bacteria.
The exposure occurred in Atlanta at the weekend and no-one has yet shown any symptoms, the CDC has said.
The FBI has told the BBC it is helping the CDC to investigate.
"This should not have happened," Dr Paul Meechan, director of the environmental health and safety compliance told the Reuters news agency. "We're taking care of it. We will not let our people be at risk."
Dr Meechan said it was too early to determine whether the transfer was accidental or intentional.
Symptoms of anthrax exposure include skin ulcers, nausea and vomiting and fever, and can cause death if untreated.
What is anthrax?
Anthrax bacteria live primarily in inactive spores and are found naturally in the soil, but people can ingest or inhale spores, which can make the anthrax active.
Not all will be sickened when exposed to anthrax but left untreated, anthrax illness can turn very serious or lead to death.
A blood culture test is used to confirm anthrax exposure.
Anthrax entered the US national consciousness in 2001, when shortly after the 9/11 attacks, letters containing powdered anthrax arrived at news organisations and the offices of US senators. Twenty-two people were sickened and of those, five people died.
Sources: FBI Anthrax investigation; CDC, San Francisco Department of Public Health
In a statement, the CDC said the anthrax samples were moved from a high-security lab to a lower-security one at their Atlanta headquarters.
"Workers, believing the samples were inactivated, were not wearing adequate personal protective equipment while handling the material," the agency said.
"The unintentional exposure was discovered June 13 when the original bacterial plates were gathered for disposal and B. anthracis colonies [live bacteria] were found on the plates."
Although the CDC believes about seven researchers had direct contact with the anthrax it is casting a wide net in who may have been exposed.
About 75 individuals are being offered a course of the antibiotic ciprofloxacin as well as an injection with an anthrax vaccine.
The normal incubation period for anthrax illness can take up to five to seven days, Dr Meechan told Reuters, but there have been cases of illness occurring as much as 60 days after exposure.