The discovery of the vials prompted a lockdown of the facility, which has since been lifted. The FBI and CDC launched investigations, which remain ongoing.

“There is no indication that anyone has been exposed to the small number of frozen vials. The frozen vials labeled ‘Smallpox’ were incidentally discovered by a laboratory worker while cleaning out a freezer in a facility that conducts vaccine research in Pennsylvania,” a CDC spokesperson told Yahoo News.

“CDC, its Administration partners, and law enforcement are investigating the matter, and the vials’ contents appear intact. The laboratory worker who discovered the vials was wearing gloves and a face mask. We will provide further details as they are available,” the spokesperson said.

Smallpox, a disease caused by the variola virus, is considered so deadly that only two labs in the world are authorized to store samples of the virus, including one in Russia and the other at the CDC in Atlanta.