The National Institutes of Health (NIH) revealed Tuesday that six decades-old vials of the smallpox virus were discovered earlier this month at a government laboratory in Bethesda, Md.
The extremely rare discovery, which officials insisted did not pose a threat to public health, comes 35 years after the deadly and highly infectious virus was eradicated, and raises questions about safety rules in federal research facilities.
U.S. officials are now conducting tests to determine whether the viral samples are active and could grow in human tissue. After that, according to the NIH, the samples will be destroyed, with the World Health Organization invited to observe.
Tuesday's announcement took place nearly one week after the vials, which appear to be from the 1950s, were first discovered in a cold storage room on the NIH campus.
They passed through a containment lab in Bethesda on their way to the official repository in Atlanta, where they were delivered late Monday night, the NIH said.
The institute said that there is "no evidence" that any of the vials were breached, nor signs that lab workers or the public were at risk of exposure.
Nonetheless, the discovery is likely to prompt investigations into how unauthorized samples of the virus were able to remain unnoticed on government property for so long.
The existence of the vials violates the international agreement designed to limit smallpox, which is considered a potential terrorist threat. The virus is permitted only for study in the Atlanta lab and at a similar facility in Russia.
Scientists found the samples while preparing to move the laboratory to the Food and Drug Administration campus in nearby Silver Spring, Md.
A total of 16 vials were discovered, according to ABC News, but only six tested positive for the smallpox virus.