A US laboratory technician is being monitored after possibleexposure to the Ebola virus, the Centers for Disease Control and Prevention says.
It said the technician was working at the CDC in Atlanta when an Ebola sample was mistakenly moved from one laboratory to another.
The employee will be monitored for 21 days. A CDC spokeswoman said there was no risk to the public.
The CDC's director said he was "troubled" by the incident.
"I have directed that there be a full review of every aspect of the incident and that CDC take all necessary measures," said Tom Frieden.
"No risk to staff is acceptable, and our efforts to improve lab safety are essential - the safety of our employees is our highest priority."
Ebola spreads through direct contact with bodily fluids. It has killed more than 7,500 people in West Africa, mainly in Guinea, Liberia and Sierra Leone.
Monitoring the technician is at least the second time this year the CDC has had to take precautionary measures after a laboratory safety problem.
In June, more than 50 CDC workers took precautionary antibiotics after being potentially exposed to a deadly strain of anthrax.