A lab worker at a vaccine research facility in Pennsylvania has stumbled across multiple vials labeled “smallpox” while cleaning out a freezer. Smallpox is so dangerous that only two facilities in the world are permitted to keep samples of it—one in Russia and the other in Atlanta. But the Centers for Disease Control and Prevention confirmed Tuesday that vials with a smallpox label were discovered at the Pennsylvania facility. “There is no indication that anyone has been exposed to the small number of frozen vials,” the CDC said in a statement. “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.” Yahoo News reported the discovery caused a lockdown at the facility, but that has now been lifted. The FBI and CDC are investigating. In 2014, six forgotten vials of smallpox were found in a cardboard box at a government scientist at a research center near Washington.