Center for Disease Control Director Dr. Thomas Frieden (L) and Dr. Anthony Fauci (R), the director of NIH's National Institute of Allergy and Infectious Diseases approach the witness table before testifying at the Senate Appropriations Committee on the US government response to the Ebola outbreak in Washington November 12, 2014. [Photo/Agencies]

ANTHRAX INCIDENT

The mishap resembled the anthrax incident, in which researchers mistakenly believed they had transferred an inactivated sample of bacteria to a lower-security lab where workers wear less-protective gear. No illnesses resulted from that breach.

Then as now, the CDC temporarily halted the transfer of samples at its high-security labs while it reviewed its protocols.

In July an agency scientist, Dr. Michael Bell, was appointed to a new role overseeing lab safety and a panel of independent experts was formed to advise the institute on such issues.

Bell has since returned to his previous post, Reynolds told Reuters, though she did not say whether anyone else had assumed the lead role for lab safety.

"I am troubled by this incident in our Ebola research laboratory in Atlanta," the CDC director, Dr. Tom Frieden, said of the latest error. "Thousands of laboratory scientists in more than 150 labs throughout CDC have taken extraordinary steps in recent months to improve safety."

The CDC also was criticized by some for not doing more to prepare the US medical establishment to deal with Ebola when a Liberian man visiting Dallas in October was diagnosed with the disease after initially being turned away from a hospital emergency room there.

Two nurses who treated that patient before he died ended up contracting the virus but survived. They are the only two people known to have been infected on US soil during the current Ebola epidemic, which has killed more than 7,500 people, most of them in West Africa.