Federal Bureau of Investigation
Federal Bureau of Investigation: Amerithrax or Anthrax Investigation
Extremely interesting site from the FBI detailing the findings of linguistic and behavioral assessments of the anthrax letter writers. Great primary source.
Council of Foreign Relations
Council on Foreign Relations: The Anthrax Letters
History of the anthrax attacks of 2001 in a Q&A format. Very concise and informative. The basics of the attack are covered. Good place to start research.