A technician at a government laboratory is being monitored for exposure to Ebola after material that may have contained a live virus was mistakenly moved, officials at the Centers for Disease Control and Prevention said Wednesday.

In a statement emailed to reporters, the CDC in Atlanta acknowledged that an error was made in moving the sample from one biosafety laboratory to another. The material was moved from a BSL-4 lab, the highest level of security for the contagious virus, to a BSL-2 lab on a sealed plate. But the move should not have been done at all, the agency said.

The CDC said one person, who has no symptoms, is being monitored for 21 days, the normal period to watch for development of illness after a possible exposure. Others who entered the lab will also be assessed, the agency said, but there is no indication that anyone else needs to be monitored.

The material never left a laboratory, so there is no danger to the general public, the agency said. The virus has killed more than 7,500 people, mostly in West Africa.

The CDC said it is investigating the event, which was also reported to Secretary of Health and Human Services Sylvia Mathews Burwell.

“I am troubled by this incident in our Ebola research laboratory in Atlanta,” CDC Director Thomas Frieden said in a statement. “We are monitoring the health of one technician who could possibly have been exposed, and I have directed that there be a full review of every aspect of the incident and that CDC take all necessary measures.

“Thousands of laboratory scientists in more than 150 labs throughout CDC have taken extraordinary steps in recent months to improve safety. No risk to staff is acceptable, and our efforts to improve lab safety are essential – the safety of our employees is our highest priority,” he said.

The worst outbreak of Ebola in history is underway in West Africa, where about 19,500 cases have been reported.

Ebola became an object of fear in the United States after Thomas Eric Duncan, a Liberian visiting the U.S., died from the virus in a Dallas hospital in October. Two of his nurses, Nina Pham and Amber Joy Vinson, contracted the virus, but both were successfully treated.

A fourth case of Ebola was diagnosed Oct. 23 in New York City. The patient, physician Craig Spencer, had just returned from working with Doctors Without Borders in Guinea. He also was successfully treated.