The article failed to mention the last woman who died, the Vietnamese woman. She got no letter that we know of and there is no anthrax in her home. The perp could have infected her directly as a test of how to infect people *without* doing it by letter.
It is not hard to imagine that the perp would now feel emboldened to pick ordinary Americans as targets and infecting them by other means than a letter. He experimented with one ( the Vietnamese lady who died) Now he may be working out a way to infect many more. And it could be that he is waiting for people to forget the urgency that was created by tests 1,2,3 and 4. Plus people will be busy and distracted during the holiday season. Less inclined to bother going to the doctor and thinking they could not have anthrax because they are not in the media and don't work for the post office.
The lesson here is if you get flu-like symptoms don't try to tough it out. Bug your doctor even if you feel stupid doing it.
And, just about any antibiotic works on the strains that were used... the key problem being in getting the antibiotics before symptoms really set in.
Other than that, it doesn't hurt to think outside the box on it. The more ideas the merrier.
Note that one of the lines was "We have this anthrax." Not "I have this anthrax", some anthrax . . . having used 1st person plural rather than lst person singular as one would expect a loner to do.
I'm sure they are looking into every aspect of the Vietnamese woman's life, including how she came to be on the Embassy roof in Saigon when it was evacuated.
I read that she paid her bills by money order. I would think that when money orders are obtained in bulk and distributed to different outlets, such as convenience stores, there ought to be some sort of serial numbers on them, and some sort of accounting procedure, even crude, for where each batch went for sale. Surely they have looked into this angle as well.
Besides that, no idea, the possibilites are enormous over who is doing this or why, there are hundreds of possibilities. Motive is always the most important I think, and in this case who "could" do it is still too large a number. narrow down the motives, all the motives that are possible, not just the most popular ones. and motives come from "who profits", and what might actually constitute a "profit" in this case.