As many as 75 scientists working in U.S. federal government laboratories in Atlanta may have been exposed to live anthrax bacteria and are being offered treatment to prevent infection, the U.S. Centers for Disease Control and Prevention said on Thursday.

The potential exposure occurred after researchers working in a high-level biosecurity laboratory at the agency's Atlanta campus failed to follow proper procedures to inactivate the bacteria. They then transferred the samples, which may have contained live bacteria, to lower-security CDC labs not equipped to handle live anthrax.

Dr Paul Meechan, director of the environmental health and safety compliance office at the CDC, said the agency discovered the potential exposure on the evening of Friday, June 13, and immediately began contacting individuals working in the labs who may have unknowingly handled live anthrax bacteria.

"No employee has shown any symptoms of anthrax illness," Meechan told Reuters.

Meechan said the normal incubation period can take up to five to seven days, though there are documented cases of the illness occurring some 60 days after exposure.

Meechan said as many as seven researchers may have come into direct contact with the live anthrax. But the agency is casting as wide a net as possible to make sure all employees at the agency who may have walked into any of the labs at risk are being offered treatment.

Around 75 individuals are being offered a 60-day course of treatment with the antibiotic ciprofloxacin as well as an injection with an anthrax vaccine.

Meechan said it is too early to determine whether the transfer was accidental or intentional. He said that all employees who were doing procedures to inactivate the bacteria were working in a biosecurity laboratory and were "tier one select agent approved," meaning they had undergone a security reliability review and deemed to be "stable, trustworthy individuals."

Meechan said the CDC is conducting an internal investigation to discover how the exposure occurred and said disciplinary measures would be taken if warranted.

"This should not have happened," he said. For those exposed, he said, "We're taking care of it. We will not let our people be at risk."