A failure to follow established safety practices caused the possible exposure, the agency said Thursday in a statement, adding that it believed the risk of infection was very low and that nobody else was at risk of exposure.
An initial CDC review showed the lab was preparing samples of Bacillus anthracis, or anthrax, for research seeking new ways to detect pathogens, but that the lab did not "adequately inactivate the samples" before sending them along, the agency said.
The protocol requires waiting 48 hours and then testing to see if spores have grown, but the lab waited only 24 hours, a CDC spokesman said. Further investigation might reveal more problems, he said.
Workers in the labs conducting the research, believing they were handling inactive B. anthracis, were not wearing gear that would adequately protect them from the live bacteria, the CDC said.
The problem was discovered Friday when live anthrax colonies were found on bacterial plates, the CDC said.
Anthrax can produce toxins in the body that cause severe illness or death, but it is not contagious, the agency said.
Anthrax became well-known to many Americans shortly after the Sept. 11 terrorist attacks. Anonymous anthrax letters mailed to members of Congress and the National Enquirer offices in Florida killed five people. The attack disrupted mail delivery and briefly paralyzed parts of the federal government.
Years later, federal investigators said that Bruce Ivins, a civilian microbiologist who had worked for the military, had acted alone to perpetrate the anthrax mailings. Ivins had killed himself eight days before the announcement.