ing, purifying, or drying the spores found in the letters. In discussions with the FBI at the January 2011 meeting, the FBI stated that some of its consulting experts referred to the letter preparations as being of “vaccine quality”, which narrowed the list of potential suspects. Nonetheless, the Bureau investigated individuals without regard to their specific skill sets. The FBI further stated that the time for preparation and equipment used in preparation of the letter materials was difficult to ascertain because of numerous variables. However, FBI officials indicated that inferences about required skills or time for spore preparation were never the sole criterion for eliminating suspects (FBI/USDOJ, 2011).

With regard to cultivation of the spores in liquid versus solid (i.e., agar-based) medium, assays for the presence of residual agar in the letter material were inconclusive, as described below. The DOJ Amerithrax Investigative Summary (USDOJ, 2010) describes the potential use of either a fermentor or an incubator with shaken flasks and liquid media. That document also suggests that a minimum of 500 ml of liquid culture would be required to produce the spores in the letters but then states “we cannot say with certainty how much material was used in the letters.”

Given the information available on the number of spores believed to have been placed in the letters and knowledge of spore yield from various types of cultivation methods, a range of required culture volumes can be estimated. Four spore-containing letters were recovered and evidence indicates that one additional letter was sent to American Media, Inc. (AMI) (Cole, 2009). A high estimate for the total number of spores sent through the mail would include five letters, each containing 1 gram of spore-containing powder with 2 × 1012 spores per gram, for a total of 1.0 × 1013 spores. A low estimate for the total number of spores sent through the mail would include five letters with 0.8 gram of spore-containing powder per letter. Two of the letters (Leahy and Daschle) might have contained 2 × 1012 spores per gram while the others (New York Post, Brokaw, and AMI) might have contained 2 × 1011 spores per gram (FBI Documents B1M2D1, B1M2D3, B1M2D6), for a total of 3.7 × 1012 spores (see Table 4-1).

TABLE 4-1 Estimated Ranges of Total Number of Spores

Total spores contained in all letters Number of letters, gram spores per letter, and spores/gram
Low estimate = 3.7 × 1012 2 letters with 0.8 gram spores per letter and
2 × 1012 spores per gram
3 letters with 0.8 gram spores per letter and
2 × 1011 spores per gram
High estimate = 1.0 × 1013 5 letters with 1 gram spores per letter and
2 × 1012 spores per gram


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