Coating an anthrax spore with Bentonite is a technique associated with the Iraqi biological weapons programme.

BBC: Q&A: The anthrax mystery

This "recipe" for the preparation of anthrax as a weapon - using silica to perform a similar function as Bentonite - is a technique associated with US experiments in the 1960s with anthrax as a weapon.

BBC: Q&A: The anthrax mystery

The trench was filled with bentonite slurry.

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Tests carried out by the US Army's Medical Research and Materiel Command Center on the anthrax spores sent to the office of Senate Majority Leader Tom Daschle and the New York Post show that the spores were not mixed with Bentonite.

BBC: Q&A: The anthrax mystery