The number of health workers potentially exposed to anthrax has risen to 84, the US Centers for Disease Control and Prevention (CDC) has said.
The count of those at risk of illness has risen from the 75 initially reported on Thursday.
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, the CDC has said.
The FBI has told the BBC it is helping the CDC to investigate.