Sixteen vials of variola, commonly known as smallpox, were found in a warehouse in Bethesda, Maryland, a statement from the Center for Disease Control and Prevention revealed Tuesday.

Employees of the National Institutes of Health last Tuesday found the vials, which were appropriately labeled “variola,” in an unused part of a storage room of a laboratory that had been used by the Food and Drug Administration since 1972.

The CDC statement confirmed there is no evidence the vials were breached and no employees were exposed to the disease.

The vials were immediately put in a containment lab and have since been flown to the CDC’s high-containment facility in Atlanta, one of only two official smallpox repositories selected by the World Health Organization. The other is in Novosibirsk, Russia.

In Atlanta, the vials will be tested to see if the virus is still alive, that is if it still has the ability to grow and infect. The CDC says the testing period will take two weeks after which the vials will be destroyed.

The CDC has invited the World Health Organization to oversee the destruction of the vials and to participate in the investigation into how the vials came to be in the warehouse and unattended for so many years. The FBI and CDC’s Division of Select Agents and Toxins will also be involved in the investigation.

“We’re trying to find out,” CDC spokesperson Tom Skinner told the Washington Post. “This certainly is an unusual event.”

Smallpox can be transmitted by any direct contact with an infected person or object such as bedding. Symptoms, which usually take 12 to 14 days to show up, begin with those associated with flu, followed by a rash in the mouth and on the skin that turn into pustules.

A worldwide vaccination program successfully eradicated the disease several decades ago, or so health officials thought. The vials appear to be from the 1950s, which is around the time of the last smallpox outbreak in the U.S. in 1949.

The last outbreak in world was in Somalia in 1977.