Authorities in the US have raised the number of employees at a government science lab who were accidentally exposed to anthrax to 86, up from the initial count of 75.
The count "may go up more as more come forward," Centers for Disease Control and Prevention spokesman Tom Skinner told AFP.
The workers at the Atlanta headquarters of the CDC have been given antibiotics and are being monitored for signs of illness.
Samples of Bacillus anthracis (anthrax) were being prepared for research at lower-security labs.
"However, the lab used a procedure that did not adequately inactivate the samples."
Therefore, workers were not wearing the proper protective equipment when they handled the samples.
"The unintentional exposure was discovered June 13 when the original bacterial plates were gathered for disposal and B. anthracis colonies (live bacteria) were found on the plates," the CDC said.
The CDC said the staff members at risk were being monitored out of an abundance of caution, though the risk of infection was "very low."
"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," it said.
Anthrax is a disease caused by a germ that lives in soil, and gained notoriety after a spate of US mail attacks in 2001 killed five of 22 people infected.