A small vial containing a rare but potentially deadly virus called "Guanarito" that could be used in a bio-terror attack has gone missing from a maximum-security University of Texas research laboratory.
The Guanarito virus is native to Venezuela and is transmitted by rodents. It causes internal bleeding and organ damage that is fatal for up to 30 percent of the people who contract it.
Officials at the lab are scrambling to explain what could have happened to it.
"There's really no possibility of anything leaving the lab in a viable form unless it is taken out intentionally," said Scott Weaver, scientific director of the Galveston National Laboratory, according to USA Today.
"It's an obscure virus. Even among the scientific community, a lot of people haven't heard of this virus," he added.
"It just doesn't seem like the virus anybody would want to steal."
The vial - which contains less than a quarter of a teaspoon of the virus - was found to be missing from a locked freezer during a routine internal inspection of the lab last week.
The facility, owned by the University of Texas Medical Branch (UTMB) houses some of the deadliest viruses in the country including ebola, anthrax and plague, and has strict security measures.
Since the virus can be inhaled and there is no known treatment or vaccine, scientists have to wear protective gear that resembles space suits and have a decontamination shower after leaving the lab, Weaver said.
Galveston and other research labs use bio-terror agents to develop vaccines and treatments.
People have become infected with the Guanarito virus by breathing dusty soil particles contaminated by rodent droppings, according to USA Today, and about one-third of those sickened will die from haemorrhagic fever.
"This is clearly an incident that is very discomforting and embarrassing to the University of Texas Medical Center and their national biosecurity lab that they have there," said William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center, in an interview with ABC News.
"You can be sure there are a lot of sweating people down the chain at that institution," he said.
Researchers believe the vial was destroyed "during normal laboratory sterilisation practices", according to a statement from UTMB President David Callender.
The statement also said the Centers for Disease Control and Prevention was notified immediately after the vial was discovered missing.
There were no indications of security malfunctions or unusual access to the lab since the last inventory in November, when the vial was present, according to USA Today.