Experiments with deadly viruses such as Ebola have to be performed in biosafety level or BSL-4 laboratories, for the highest level on containment. (Tim Brakemeier/AFP/Getty Images)

Researchers studying Ebola in a highly secure laboratory mistakenly allowed potentially lethal samples of the virus to be handled in a much less secure laboratory at the Centers for Disease Control and Prevention in Atlanta, agency officials said Wednesday.

One technician in the second laboratory may have been exposed to the virus and about a dozen other people have been assessed after entering the facility unaware that potentially hazardous samples of Ebola had been handled there.

The technician has no symptoms of illness and is being monitored for 21 days. Agency officials said it is unlikely that any of the others who entered the lab face potential exposure. Some entered the lab after it had been decontaminated. Officials said there is no possible exposure outside the secure laboratory at CDC and no exposure or risk to the public.

“At this time, we know of only the one potential exposure,” CDC Director Tom Frieden said in a telephone interview.

The mistake took place Monday afternoon. It was discovered by laboratory scientists Tuesday and within an hour reported to agency leaders. The error, which is under internal investigation, was reported to Secretary of Health and Human Services Sylvia Mathews Burwell and to a program that has oversight over pathogens such as Ebola and anthrax.

View Graphic This is the largest outbreak of the Ebola virus in history.

The mistake comes after a series of incidents this summer involving the mishandling of dangerous pathogens at the nation’s labs. An incident at a CDC lab in June potentially exposed dozens of employees to live anthrax because employees failed to properly inactivate the anthrax when transferring samples.

In a statement, Frieden said he was troubled by the mistake in the CDC’s Ebola research lab. “Thousands of laboratory scientists in more than 150 labs throughout CDC have taken extraordinary steps in recent months to improve safety,” he 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.”

The CDC operates advanced biosafety laboratories in which dangerous pathogens such as Ebola can be handled by investigators who wear biohazard suits that keep them from being exposed. But what happened this week illustrates the impossibility of eliminating human error from even a state-of-the-art facility.

In the interview, Frieden said the agency’s goal to have systems in place to mitigate human error “was not met here.”

The mistake was discovered Tuesday when workers looked in the freezer in the Ebola research lab — one of the highest-security biosafety labs, known as a Level 4 — and saw material that was supposed to have been sent to a different, less-secure lab in the same building.

They realized something was wrong.

The researchers had been studying the effects of Ebola on guinea pigs in the high-security lab to find out whether the Ebola strain that has devastated West Africa this year is deadlier than previous strains.

But there was a mix-up this time: Less-hazardous material that should have gone to the second lab down the hall was placed in the first lab’s freezer. The hazardous material, which possibly contained live Ebola virus, was put in a spot to be transferred to the second lab, CDC officials said.

The technician in the second lab should have recognized, via the color coding on the test tubes, that this was hazardous material that should have stayed at the first lab, officials said. That technician is the person who could have been exposed.

The lab where Monday’s potential exposure occurred was decontaminated and the material destroyed as a routine procedure before the error was identified. The laboratory was decontaminated for a second time and is now closed. Transfers from the high-security lab have stopped while the review is taking place.

The high-security lab where the first mistakes were made also performs diagnostic tests for Ebola, and has conducted hundreds of those tests since July. Stuart Nichol, a top CDC official, said diagnostic testing for Ebola will be moved to a different lab.

The latest incident comes at a time when the CDC is taking a leading role to fight the epidemic that has killed more than 7,500 people in Guinea, Liberia and Sierra Leone, and to improve domestic preparedness against Ebola. More than 170 CDC infectious disease specialists are in West Africa. Frieden returned late Saturday from his second trip to the region.

After the summer’s incidents, the CDC temporarily banned transfer of all biological materials from its labs, conducted a wide-ranging safety review, appointed a new director of lab safety and created an outside lab safety advisory group. The CDC labs conduct some of the world’s most sophisticated research into infectious diseases.

At congressional hearings, Frieden vowed to improve the agency’s overall safety culture as well as put in place stronger oversight measures.

In the June incident involving live anthrax, more than 80 workers may have been exposed after employees unknowingly sent samples of the bacterium from one CDC lab to other CDC labs. During the anthrax investigation, agency officials learned about several other instances in which deadly pathogens had been improperly sent to other laboratories over the past decade.

No one became infected or fell ill in those incidents, and all the organisms were safely disposed of, officials said.

The most serious of those previously undisclosed incidents took place in March, when a CDC lab in Atlanta sent a sample of flu virus contaminated with the deadly H5N1 influenza virus, a much-feared bird flu strain, to a U.S. Department of Agriculture laboratory in Athens, Ga. CDC staff members failed to report the incident to top leaders at the time.

Biosafety experts have criticized the lack of coordination and oversight at laboratories inside and outside the federal government that conduct research on microbes that could be used as bioterrorism agents.

“Such events like this are absolutely unacceptable even once. When they occur multiple times, you have to ask yourself what systems are in place to prevent this from happening,” Michael Osterholm, director of Center for Infectious Disease Research and Policy at the University of Minnesota, said in an interview Wednesday.

“To err is to be human. We expect that to happen in any kind of high-tech setting,” Osterholm said. “So what you do is build in a set of procedures and checks and balances. They need to be in place to account for human error.”