Thanks to a comment, I just reread Don Foster's The Message in the Anthrax, published in Vanity Fair and Reader's Digest in 2003. This is a must-read if you want to understand the universe of evidence that has accrued in this case. Foster discusses the hoax letters; the Assaad story (by Laura Rozen); how the FBI ignored or misinterpreted evidence; the makeup of the powder; and the circumstantial case against Hatfill. He reminds us that there were at least 7 letters sent, based on who came down with anthrax, though only four were recovered. (Which brings new questions to mind: what forensic studies were done on the anthrax collected from the sites at which no letter was recovered? What motives have been proposed for attacking these other offices?)
It will give you plenty to chew on.