The agency "cannot rule out the possible exposure" of one of the lab technicians, the statement reads. This technician had no symptoms and will be monitored for 21 days.
Additional employees who entered the lab will also be assessed, the statement reads, but it doesn't say how many.
CDC workers reported the incident to leadership "within an hour" of its discovery, according to the statement. The CDC is conducting an internal investigation.
"Thousands of laboratory scientists in more than 150 labs throughout CDC have taken extraordinary steps in recent months to improve safety," Frieden said. "No risk to staff is acceptable, and our efforts to improve lab safety are essential -- the safety of our employees is our highest priority."