Without question.
They had a direct connection to the AMI building (the landlord who leased them their apartment). The Boca Raton postal facility was contaminated. They were seen by a doctor and treated for anthrax symptoms. The letters that were recovered were written in a style compatible with English as a 2nd language.
...And Bob Stevens October 5th death points to an attack originating prior to 9/11/2001.
Moreover, the anthrax from the recovered letters was weaponized and had only a single non-anthrax component: silica...whichs points to a professional clean-room lab operation.
So timing, motive, the weapon itself, personal connections, handwriting evidence, geography, processes involved...it all points to the 9/11 attackers as being behind the anthrax attacks.
Remember, it wasn't a uniform mixture ~ different samples had different silica content.
Simply cleaning the bacterial mat of debris and producing spores "weaponizes" anthrax quite nicely.