'Domestic' anthrax was probably stolen by foreign students from University and Government labs who use these students as a cheap labor pool. They are the ones with the most probable access and the most likely motivation.
1. As you stated, Radical-enviros are much more likely to have access to bio equipment and labs (read Universities).
2. They profess an obligation to radically reduce population.
3. Anthrax fits them better than other methods: It doesn't damage the environment, and it's a natural-occurring substance. Additionally, because it is not contagious between humans, the possibilities of artificially induced genetic mutations (distortions of nature) are very low.
4. Enviro-Rads profess no inherent value in individual human life.
Anyone got anything to add to this list?