It added in a statement that procedures used in two of those laboratories in Atlanta, where the C.D.C. is based, may have “aerosolized the spores,” essentially blowing the bacteria into the air. The exposure was discovered June 13, when the bacterial plates were collected for disposal and live B. anthracis colonies, or anthrax bacteria, were found.

“The likelihood that anyone was actually exposed is very small,” Mr. Skinner said.

Anthrax infects humans by touch, by inhalation or by consuming it. The inhaled form is the most dangerous, and among the 18 such cases identified in the United States during the 20th century, the fatality rate was around 75 percent, according to the C.D.C.’s website. After the terrorist attack in fall 2001 in which B. anthracis spores were released through the mail, five of the 11 people who were ill died.

The incubation period for anthrax is usually more than two weeks, so it is possible the scientists could still come down with symptoms. The incubation period can sometimes take months, according to the agency’s website.

Lab and hallway areas were being decontaminated after environmental sampling was conducted, the agency said. It added that the areas would be reopened when agency officials considered it safe to operate. The agency said that it “believes that other C.D.C. staff, family members and the general public are not at risk of exposure and do not need to take any protective action.”

The error arose, according to Paul J. Meechan, the agency’s director of health and safety, as scientists were testing a new way to kill anthrax bacteria with chemicals instead of radiation.