The 41-year-old has been charged with possessing a biological agent with intent to use as a weapon.
Experts say the United States is far better able to identify a biological agent that it was just two years ago.
James Everett Dutschke of Tupelo, Mississippi, has been charged with possession and use of a biological agent in connection with that case.
Security experts feared someone might be able to release a biological agent into the air stream that flows into the building, sources said.
"Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin, " according to the bureau.
According to an FBI news release, Mr Dutschke was charged with "knowingly developing, producing, stockpiling, transferring, acquiring, retaining and possessing a biological agent, toxin and delivery system, for use as a weapon, to wit: ricin".
Tupelo, Mississippi (CNN) -- A Tupelo, Mississippi, man has been charged with possession and use of a biological agent as a weapon in connection with an investigation into ricin-tainted letters sent to President Barack Obama and others, federal authorities said Saturday.
But our information is, as everybody knows, never complete about a subject like that - never perfect, and the idea that we didn't have, you know specific proof that he was planning to give a biological agent to a terrorist group doesn't really lead you to anything because you wouldn't expect to have that information even if it were true.
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Now we find ourselves in the midst of a conflict unlike any we've ever known, faced with the possibility that terrorists could smuggle a deadly biological agent or a nuclear weapon into the middle of one of our own cities.
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If a chemical or biological agent binds to the electrode, it triggers a signal.
Anthrax, which in nature is spread through infected animals and whose spores can live in the soil for years, is considered a potential agent for use in biological warfare.
James Woolsey, his entire suspected inventory of the biological agent anthrax would fill roughly half a standard semi's tractor trailer.
Biological weapons experts said the biological agent is extremely difficult to turn into a weapon of mass destruction.
CNN: U.S. officials still on 'high alert' for additional attacks
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