Is the FBI intentionally keeping the wraps on this story as some speculate? The Federation of American Scientists have put together an analysis of the letters which is very interesting. A quote from the introduction:
The perpetrator is surely too smart to believe that either the FBI’s ludicrous recent actions or the White House protestations of ignorance mean that the authorities are not on to him. Blanketing Central New Jersey with fliers showing handwriting that was obviously disguised can’t possibly evoke useful information, nor can letters to 32,000 American microbiologists, 31,800 of whom live in a different world from the perpetrator. This is no way to instill public confidence in the competence of the FBI. [...] Most importantly, the apparent lack of action is sending a dangerous message to potential bioterrorists.