this information, one cannot rule out the intentional addition of a silicon-based substance to the New York Post letter, in a failed attempt to enhance dispersion. The committee notes that powders with dispersion characteristics similar to the letter material could be produced without the addition of a dispersant.

Early in the investigation, AFIP performed SEM-EDX analysis of a New York Post letter sample and found regions in the sample having high silicon content but no oxygen, suggesting the presence of silicon-rich material that was not related to nanoparticulate silica. While this observation could have led to an explanation for the difference between the bulk and individual spore measurements, follow-up experiments apparently were not performed. The committee notes that this information was not made available to it or to the FBI until spring 2010.

Finding 4.4: Surrogate preparations of B. anthracis did reproduce physical characteristics (purity, spore concentration, dispersibility) of the letter samples, but did not reproduce the large amount of silicon found in the coats of letter sample spores.

Surrogate preparations by DPG, using B. anthracis from the Leahy letter as the starter source (FBI Documents, B1M13D3), reproduced the general physical characteristics of the letter samples (purity, spore concentration, dispersibility) but not the silicon chemical characteristics. Surrogate preparations showed that samples having bulk silicon content up to 5 percent could be prepared without intentional addition of silicon dispersant. However, none of the DPG surrogate preparations analyzed for silicon in the spore coat were similar to the New York Post, Daschle, and Leahy letter samples with respect to either the amount per spore of silicon incorporated in the coat or the fraction of spores observed to contain silicon in the coat. Furthermore, the committee sought, but could not obtain, a detailed explanation of the thought process that went into selection of the DPG methods or their relationship to the Buran and Abshire preparations. The committee acknowledges that there were many more possible scenarios for spore preparation than could have been feasibly explored with available resources and in a reasonable period of time. However, it was not clear to the committee how the subset of surrogate preparation methods was selected and whether these choices were based on an understanding informed by the investigation or on other assumptions about the approach taken to produce the evidentiary materials.

Finding 4.5: Radiocarbon dating of the Leahy letter material indicates that it was produced after 1998.

The spores in the letter were not taken directly from a stockpile produced many years ago. One or more recent growth steps would have been required,

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