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According to Richard Spertzel (head of the biological-weapons section of Unscom from 1994-99) in an article in the WSJ (see sig link), "The latest line of speculation asserts that the anthrax's DNA, obtained from some of the victims, initially led investigators to the laboratory where Ivins worked. But the FBI stated a few years ago that a complete DNA analysis was not helpful in identifying what laboratory might have made the product...the anthrax in this case, the "Ames strain," is one of the most common strains in the world."
DNA analysis, in other words, leads everywhere and nowhere.
Spertzel seems to suggest Russian involvement in his article, based on the limitations of USMRIID, their internal security, and the sophisticated treatment of the spores. However, his version only increases my skepticism that some foreign agent was behind the attack.
One, I don't really see any logical explanation for why would Russians (or anyone working with them or obtaining biological agents from them) would want to kill the particular targets selected.
Two, given the political use of the anthrax attacks in general and the "bentonite" story in particular, it's hard not to suspect crime and coverup.
Three, the likelihood that bioweapons development hasn't been quarantined from the politicization, deceit and incompetence rampant in government suggests someone, rogue or under orders, connected with a government lab or entity was involved operationally. To claim Americans couldn't do the job just seems ridiculous, and suspicious on its face.
Speculative, admittedly, but provocative.