(CNN) -- At least two of the vials employees at the National Institutes of Health found in an unused storage room earlier this month contain viable samples of the deadly smallpox virus, the Centers for Disease Control and Prevention said Friday.
Employees found six forgotten vials when they were preparing to move a lab from the Food and Drug Administration's Bethesda, Maryland, campus to a different location. The laboratory had been used by the NIH but was transferred to the FDA in 1972.
When the scientists found the vials, they immediately put them in a containment lab and on July 1 notified the branch of the government that deals with toxic substances, called the Division of Select Agents and Toxins.
The CDC said previously there is no evidence that any of the vials was breached, nor were any of the lab workers exposed to the virus.
On Monday, law enforcement agencies transferred the vials to the CDC's high-containment facility in Atlanta. The CDC is one of only two official World Health Organization designated repositories for smallpox.
CDC Director Tom Frieden said his scientists worked through the night on the samples as soon as they got them. Testing confirmed that there was variola DNA in the vials.
Additional test results showed "evidence of growth" in samples from two of the vials, suggesting that the smallpox virus is alive.
The other four vials still need to be tested for evidence of growth, Frieden said Friday. After their investigation is complete, the CDC will destroy the vials and all the growth that came out of them. The World Health Organization will oversee that destruction.
Smallpox, known also by its scientific name as variola, was the deadly virus that was the scourge of civilization for centuries. It's been considered an eradicated disease since 1980, following successful worldwide vaccination programs. The last known outbreak in the U.S. was in 1947 in New York.
The vials were created February 10, 1954; that is before the smallpox eradication campaign began.
Frieden says that the NIH is currently scouring their buildings to make sure there are no other surprises left in unused storerooms. He says the problem in this case is not in the creation of the vials; the discovery points to a "problem in inventory control."
CNN's Miriam Falco contributed to this story.