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Forgive me if someone has brought this up before, but wouldn't the existence of an accomplice in this crime explain the mailing schedule inconsistencies? Ivins must have handed the letters off to another guy to do the actual mailing.
Obviously the FBI would not want to broach this subject because it implies that Ivins was just the supplier, not the mailer, and therefore the real bad guy is still on the loose. Also, it means the mailer might still have a supply of anthrax to use in the future.
Or maybe I've just been watching too many episodes of Monk lately. Nevermind.