The significant issue with this anthrax was not "purity" but SIZE the spores were milled to the perfect size for inhilation. Like Baby Bear they had to be juuust right not too small not too big.
There were only a few labs capable of that kind of milling and of that few only Iraq had the motivation to do something like this.
That is totally untrue.
First of all, anthrax spores ONLY COME IN ONE SIZE. They are roughly 1 micron in diameter, and 1.5 microns in length. Milling down a spore to get it to the right size would be like milling down an ostrich egg to create a chicken egg. It's idiotic. It's impossible.
Second, the anthrax spores in the letters WERE NOT MILLED. Milling is done to break up clumps of spores which were dried together. Milling leaves traces. When you mill spores, you get chips of spores in the powder, and most of the spores would appear to have flat sides as a result of the milling. (If you have two or more spores stuck together, and you separate them by milling, the area where they were stuck together will be flat.) Everyone who looked at the spores said there was no sign of milling.
The nonsense about milling was just a misunderstanding by people who knew nothing about such things. It was probably the DUMBEST thing ever said about the anthrax, and it was said by MANY people who just didn't know what they were talking about.
Ed