Being a beleagured, American-born, Republican in South Florida, here's my theory:
Since it is difficult to cause widespread illness with anthrax without sophisticated machinery and precise weather conditions, this "envelope" (if that was the source) was sent out in hopes of spreading the disease to at least one person in a high-profile location. Since these tabloids are sold all over the US, and abroad, the American Media Building was the theater of choice.
Once that plan succeeded (and it may be the only viable spores these people have), it was, and will be, followed up by random mailing of "white powder," designed to create panic and inevitable mistrust of the government (who are supremely positioned for mistrust, after the last 8 years).
Psychological warfare, played to by the media which is ticked off about being left out of the war planning. They are angry at the government due to lack of information, and what better way to get revenge, knowingly or unknowingly, by pointing out that "the government's not telling us everything."