About 75 workers at the Centers for Disease Control and Prevention may have been accidentally exposed to dangerous anthrax bacteria this month because of a safety problem at some of its labs in Atlanta, the federal agency revealed Thursday.

Independent experts say it appears to be the largest incident involving anthrax, a potential bioterrorism agent, in a U.S. lab in at least a decade. CDC officials say the risk of infection seems very low, but the employees were being monitored or given antibiotics as a precaution.

“Based on the investigation to date, CDC believes that other CDC staff, family members, and the general public are not at risk of exposure and do not need to take any protective action,” a statement from the agency says.

The problem was discovered last Friday, and some of the anthrax may have become airborne in two labs the previous week, the statement says.

The safety lapse occurred when a high level biosecurity lab was preparing anthrax samples. The samples were to be used at lower security labs researching new ways to detect the germs in environmental samples. The higher security lab used a procedure that did not completely inactivate the bacteria.

Workers in three labs who later came into contact with the potentially infectious samples were not wearing adequate protective gear because they believed the samples had been inactivated. Procedures in two of the labs may have spread anthrax spores in the air.

Live bacteria were discovered last Friday on materials gathered for disposal. Labs and halls have been tested and decontaminated and will reopen “when safe to operate,” the CDC statement says. Because proper procedures were not followed, the agency said workers will be disciplined “as necessary.”

“It’s unacceptable and we’re going to do everything we can . . . to make sure it doesn’t happen again,” said CDC spokesman Tom Skinner.