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 may have 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 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.

The mistake comes after a series of incidents earlier 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.

Frieden is troubled

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 has advanced biosafety labs 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."

Mistake found Tuesday

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 if 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.

Color-coded test tubes

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.

After the summer's incidents, the CDC temporarily banned transfer of all biological materials from its labs, conducted a broad 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 disease.