The Federal Bureau of Investigation and the Centers for Disease Control and Prevention are investigating a discovery Tuesday evening of alleged “smallpox” vials in a Pennsylvania laboratory.

Federal officials opened the investigation after a worker from Merck, one of the largest pharmaceutical companies in the United States, discovered 15 vials, five of which had been labeled “smallpox” and 10 of which had been labeled “vaccinia,” according to Yahoo News. At the time of the discovery, the worker had been cleaning out a freezer at the facility, located just outside of Philadelphia.