I also think you are right that the person in St. Petersburg who is mailing these letters is involved in the terror campaign, regardless of whether his particular letters happen to carry anthrax. Even this latest letter, the one to the local paper, was mailed before Brokaw was identified publicly as a target. To believe that this was just a coincidence would imply that the mailer sent a threat referencing anthrax ("1st case of disease now blow away this dust so you see how the real thing flys") and a threatening letter to Brokaw, who was, unbeknownest to the public, already the target of an anthrax-laced letter, by chance. That is, on the face of it, fantastic.