A laboratory mistake at the Centers for Disease Control and Prevention in Atlanta may have exposed a technician to the deadly Ebola virus, federal officials said on Wednesday. The technician will be monitored for signs of infection for 21 days, the incubation period of the disease.

Word of the accident provoked concern and disbelief from some safety experts. Dangerous samples of anthrax and flu were similarly mishandled at the C.D.C. just months ago, eroding confidence in an agency that has long been one of the most respected research centers in the world.

Other employees who entered the lab where the mistake occurred were being examined for possible exposure. There are fewer than a dozen, and so far it appears that none were infected, said Thomas Skinner, a C.D.C. spokesman.

The samples were properly contained and never left the C.D.C. campus, so there is no risk to the public, officials said.