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 these 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 really unfortunate that this happened. It’s unacceptable and we’re going to do everything we can to understand why it happened and what we need to do differently to make sure it doesn’t happen again,” said CDC spokesman Tom Skinner.

He said the incident was revealed on Thursday, nearly a week after it was discovered, because the first priority was investigating the extent of the problem and notifying workers.

“When we learned what had happened we moved as swiftly as possible to contact anyone who was possibly exposed,” he said.

Skinner said he did not know how many employees were taking antibiotics or how they were exposed.

Anthrax infections can occur through skin contact but “if you inhale it and you get it in the lungs, that’s a lot more dangerous,” said Paul Roepe, an infectious disease specialist at Georgetown University Medical Center. The ability of antibiotics to prevent infection depends on how quickly they are started, he said.

Anthrax created fear in 2001, when five people died and 17 others were sickened from letters containing anthrax spores sent through the mail. The FBI blames the attacks on a lone government scientist, Bruce Ivins, who committed suicide.

Scott J. Becker, executive director of the Association of Public Health Laboratories, said this appears to be the largest potential anthrax exposure in a lab since then, and he urged the CDC to fully disclose the results of its investigation.

“It’s important to learn what happened there so we can ensure it doesn’t happen again,” he said. Labs “work on anthrax all the time,” and the CDC’s statement seems to suggest human error, “not a system failure.”

“They’re taking all the necessary steps” for potentially exposed workers, he added.