[DBrow]

It could have been one or more of the 9/11 terrorists, or someone working with them.

But FBI never likes to admit to terror ops in the conus, look at WTC I, where top government officials were thinking that the truck bomb was "a transformer fire" for a day or so.

[LambSlave]

Here is a somewhat dated article that describes the challenges of weaponizing Anthrax and why the investigation shifted to the US labs; the identity and proportion of the chemical agents used to prevent clumping are how the origin of the weaponized strain is determined.