Up to 75 scientists and staff in US government laboratories in Atlanta may have been exposed to live anthrax bacteria after researchers failed to follow safety procedures, prompting an investigation by federal authorities.
Researchers working in a high-security bioterror response lab at the US Centers for Disease Control and Prevention were preparing inactivated samples of the deadly organism, the CDC said.
But the bacteria may still have been infectious when the samples were transferred to lower-security CDC labs not equipped to handle live anthrax.
Two of the three labs conducted research that may have aerolised the spores, the CDC said.
The agency first detected the exposure on June 13, when live bacteria were found on the original slides used by scientists. Environmental sampling was done and the lab areas remain closed for decontamination.
"No employee has shown any symptoms of anthrax illness," Dr Paul Meechan, director of the environmental health and safety compliance office at the CDC, told Reuters.
"This should not have happened," he said. For those exposed, "we're taking care of it. We will not let our people be at risk."
The safety breach in the nation's premier bioterror lab raises new doubts about security measures at the CDC, whose infection control protocols are held up as a model to the world.
The FBI is working with CDC to investigate the incident, but has no evidence of foul play, a spokesman for the bureau said.
Meechan said the CDC was conducting an internal investigation and that disciplinary measures would be taken if warranted.
The handling of pathogens inside US government laboratories has been a growing concern since 2008, when the FBI identified Dr Bruce Ivins, a US Army anthrax researcher, as the prime suspect in a series of anthrax letter attacks in 2001.
The threat of insiders having access to lethal bioterror agents led to a crackdown on lab security.
The Ivins case touched off fears that "malicious actors" would use the substances to harm humans, according to a 2009 report by the American Association for the Advancement of Science.
The CDC said that once the live bacteria were discovered, it immediately began contacting people working in the labs who may have been exposed.
Meechan said seven researchers are the most likely to have come into contact with live anthrax, but the agency is casting as wide a net as possible to offer treatment to anyone who may be at risk.
The normal incubation period for Bacillus anthracis can take up to five to seven days, though there are documented cases of the illness occurring some 60 days after exposure, Meechan said.
Infection can occur through a cut in the skin, breathing in anthrax spores or eating tainted meat.
Of the three, the biggest threat is inhalation anthrax, in which bacterial spores enter the lungs. Once they germinate, they release toxins that can cause internal bleeding, swelling and tissue death.
Inhalation anthrax occurs in two stages. In the first stage, symptoms resemble a cold or the flu. In the second stage, anthrax causes fever, severe shortness of breath and shock. About 90 percent of people with second-stage inhalation anthrax die, even after antibiotic treatment.
Meechan said CDC workers in the lower-security labs were likely not wearing masks. Around 75 people are being offered a 60-day course of treatment with the antibiotic ciprofloxacin and an injection with an anthrax vaccine.