Details of Silicon-Tin Chemistry of Anthrax Attack Spores Published; Willman Tut-tuts

Sandia National Laboratories image of attack spore. In the upper frame, silicon, in green, is found exclusively on the spore coat and not on the exosporium (outer pink border).

On Saturday, the Journal of Bioterrorism & Biodefense published an article (pdf) by Hugh-Jones, Rosenberg and Jacobsen that provides the details of their theory, first described in a McClatchy article, that the anthrax spores employed in the 2001 anthrax attacks were “weaponized” by a process that involved tin-catalyzed polymerization of silicon monomers.  Wasting no time, David Willman was quickly trotted out in the Los Angeles Times on Sunday to tut-tut this latest information as arising from “critics” of the FBI and to provide an outlet for those who unquestioningly parrot the FBI’s conclusion from its Amerithrax investigation that Bruce Ivins acted alone in carrying out the attacks.

Shortly after the McClatchy article was published, I provided this perspective on the new revelations it contained:

The presence of silicon and how it may have gotten into the anthrax material has been a point of great controversy throughout the entire investigation. This question is important because the chemical nature of the silicon and the level at which it is present is presumed to be an indicator of whether the anthrax spores have been “weaponized” to make them suspend more readily in air so that they are more effective in getting into the small passageways of the lungs of the intended targets of the attack. Early in the investigation, Brian Ross published “leaked” information that the spores had been weaponized through addition of bentonite and that Iraq had a weaponization program that used bentonite. This report turned out to be false, as no evidence for bentonite has been found. A more sophisticated type of weaponizing would rely on mixing the spores with nanoparticles of silica (silica is the common name for the compound silicon dioxide) to make them disperse more easily.

The FBI carried out a special form electron microscopy that could identify the location of the silicon in the spores from the attack material. They found that the silicon was in a structure called the the spore coat, which is inside the most outer covering of the spore called the exosporium. If silica nanoparticles had been used to disperse the spores, these would have been found on the outside of the exosporuim (see this diary for a discussion of this point and quotes from the scientific literature) because they are too large to penetrate it.  No silicon signature was seen on the outside edge of the exosporium.  What is significant about the type of silicon treatment suggested in the McClatchy piece is that both high silicon and high tin measurements were found in several samples and that there is an alternative silicon treatment that would involve a tin-catalyzed polymerization of silicon-containing precursor molecules. McClatchy interviewed scientists who work with this process and they confirmed that the ratio of silicon to tin found by the FBI is in the range one would expect if such a polymerization process had been used.

What McClatchy doesn’t mention in their report is that it would seem for a polymerization process of this sort, the silicon-containing precursor molecules would be small enough to penetrate the exosporium before being polymerized, or linked together into much larger molecules, once they reached the spore coat. This would mimic the location of silicon incorporated “naturally” into spores.

As the photo above shows, the anthrax spores in the attack material had silicon that was found exclusively in the spore coat and not in the exosporium.  This photo is taken from a news article (subscription required) published in March, 2010 in Science magazine.  I quoted the article in this diary from the same day:

A more detailed analysis by Joseph Michael and Paul Kotula of Sandia National Laboratories in Albuquerque, New Mexico, contradicted that conclusion. Studying individual spores with a transmission electron microscope, they found that the silicon was located within the spore coat, well inside the cell’s exosporium (outermost covering). By contrast, when they looked at surrogate spores weaponized with silica, the silicon was clearly outside the exosporium.

But the Sandia study, presented last September to a National Academies panel reviewing the science behind the investigation, still leaves questions. Out of 124 spores from a letter mailed to Senator Patrick Leahy of Vermont, Michael found the silicon-and oxygen signature in 97—78% of the sample. The signature was present in 66% of a sample from a letter to former Senator Tom Daschle and in 65% of spores from a letter sent to theNew York Post.

Out of nearly 200 other anthrax samples from different labs, none came close to displaying such a prominent silicon signature. The highest, in a sample from Dugway Proving Ground in Utah, was 29%. The researchers couldn’t find silicon in the coat of a single spore out of some 300 taken from RMR-1029, the flask in Ivins’s lab identified as the source of the bacteria used in the attacks; they concluded that all the silicon had come from the culture.

Note that the Sandia study found that the attack material had silicon present in the spore coats of a higher percentage of the spores than in any samples they analyzed where silicon had been incorporated into the spore coat during culture.  Note also that the only “weaponization” treatment employed in the Sandia study was the treatment of spores with silica nanoparticles which coated the exosporium rather than the spore coat.

As I had suggested after first reading the McClatchy article, the Hugh-Jones et. al. article [full citation: Hugh-Jones ME, Rosenberg BH, Jacobsen S (2011) The 2001 Attack Anthrax: Key Questions, Potential Answers. J Bioterr Biodef S3:001. doi:10.4172/2157-2526.S3-001] describes in detail the chemistry of how the silicon monomers could penetrate the exosporium prior to polymerizing on the surface of the spore coat:

All the evidence in the public domain is consistent with the concept that the spore coats of the attack anthrax were silicone-coated. Silicone polymers are typically formed by hydrolysis of a silicon compound such as dimethyldichlorosilane (or other silanes with similar substituents), which contains no oxygen. Hydrolysis replaces the chlorine atoms with oxygen to form dimethylsilanol, which polymerizes spontaneously to form polydimethylsiloxane, containing silicon and oxygen in equal amounts. The polydimethylsiloxane chains can then be cross-linked (“cured”) to form a three-dimensional silicone coating for encapsulation. This step requires an organotin catalyst such as a dibutyltin dicarboxylate.

A procedure of this kind can be envisioned for encapsulating B. anthracis spores. Silane monomers like dimethyldichlorosilane are low-molecular-weight liquids that probably can penetrate the exosporium, the loose-fitting membrane sac that encloses the spore. If silane monomers were added to a suspension of dry spores in an organic solvent, the silane would not contact moisture until it reached the spore coat, where residual moisture diffusing from the core inside the spore would cause hydrolysis, followed by polymerization at the spore coat. The polysiloxane chains that would be formed at the spore coat could then be cross-linked to encapsulate the spore. This step would require continued diffusion of moisture from inside the spore, as well as an organotin catalyst. Organotins have low solubility in water but, like silanes, are soluble in organic solvents such as ether, carbon tetrachloride, etc. The ratio of tin to silicon in the attack spores is “about right” for a tin catalyst used to produce a silicone coating,
according to a chemist in the field.

As stated previously in the McClatchy article, Hugh-Jones, et. al. point out that it would not have been possible to treat anthrax spores with this process at USAMRIID, where Ivins carried out all of his work:

It would be difficult not to conclude that the spores in the attack letters were prepared for some purpose other than terrorism. Potential procedures that might be applicable for silicone coating of spores, barely touched on here, are complex, highly esoteric processes that could not possibly have been carried out by a single individual. They would require a laboratory with specialized capabilities and expertise not found at USAMRIID, in addition to the possession of the correct strains of B. anthracis Ames associated with flask RMR 1029.

Personnel at USAMRIID all agree that no work with non-aqueous (dry or suspended in organic solvents) anthrax spore preparations is carried out there. The technological ramifications of this are that had Ivins engaged in such work, he would have encountered barriers. His need to decontaminate areas where he worked with dry spore powder would have been greater than areas where he worked with suspensions of spores in water since dry powder would be more likely to disperse over larger portions of the work area. Furthermore, there is no indication that the hot suite where Ivins worked with spores is equipped to handle organic solvents. Safe removal of volatile solvent fumes [ether fumes are responsible for the explosions and fires frequent in amateur meth labs] while still preventing release of spores would require additional air-handling technology that there would have been no reason to have at the USAMRIID hot suites if only water suspensions of spores would be present. Furthermore, the actual polymerization and curing process would be likely to generate organotin vapors that can be quite toxic if not vented properly.

In response to this publication of the details of how anthrax spores could come to have the silicon and tin content observed, even including the observed location of the silicon in the attack material, David Willman attacked this information in Sunday’s Los Angeles Times. Here is how Willman describes various recent questions that have been raised about the FBI’s data and conclusions:

One account came from three scientists — long critical of the FBI — whose questions were the subject of a story in the New York Times. Another came from the nonprofit group ProPublica,  the PBSdocumentary unit Frontline and McClatchy Newspapers. The coverage highlighted the lingering antagonism toward the FBI among some of Ivins’ colleagues at the Army‘s biowarfare research center at Ft. Detrick, Md.

In response to the reports, FBI spokesman Michael Kortan said the bureau stood by its conclusion that Ivins was the perpetrator, “based both on the scientific findings and the results of the extensive traditional criminal investigation.”

Note that Hugh-Jones, et. al. are described as “long critical of the FBI” and that USAMRIID personnel who disagree with the FBI  are painted as having “lingering antagonism toward the FBI”.  Willman then trots out an FBI spokesman to assure us that the FBI has no doubts about its work or conclusions.

Willman goes to special pains to address the silicon-tin story.  After again calling Hugh-Jones, Rosenberg and Jacobsen “longtime critics of the FBI” lest we forget that phrase, Willman goes on to try to impeach Rosenberg by pointing out that she was an early advocate of the theory that Steven Hatfill had been behind the attacks.  But Willman’s attempt to negate the silicon-tin polymerization theory falls far short of the science:

Joseph R. Michael, the investigation’s top scientist in charge of determining whether the mailed anthrax was treated with additives, acknowledged that it may never be established how tin or another common element, silicon, got into some of the spores. But Michael said that if tin or silicon had been intentionally added, it probably would have coated the exterior surfaces. He said he found trace levels of tin and silicon only inside the spores.

This is the same Joseph Michael of Sandia National Laboratories who produced the image at the top of this post.  Recall that in those experiments carried out for the FBI, Michael and his colleagues found that silica nanoparticles added to spores after they were dried resulted in the silicon signature showing up on the exosporium, rather than on the spore coat, as found in the attack material.  Michael’s work was carried out before the tin-catalyzed silicon polymerization theory was advanced.  In his quote to Willman, it’s not clear whether Michael has not read the Hugh-Jones et. al. paper and its explanation of how the silicon monomers would be expected to penetrate the exosporium before polymerizing at the spore coat or if he is just choosing to claim that such a treatment would be unlikely, so that it would be probable that exogenously added silicon or tin would be found on the exosporium.  At any rate, Willman’s quote makes Michael appear entirely unable to consider theories that conflict with his experiment that included only one among the countless number of techniques that could have been employed to introduce the silicon and tin to the attack spores.

31 replies
  1. emptywheel says:

    Interesting piece, Jim.

    So this comes down to Hugh-Jones et al providing a theory to explain one of the unexplained mysteries of the case, and Willman using Michael to refute it.

    And explain the picture above–you say it shows the chemicals in the coating, but Michael took the picture, and says he only found it inside. I don’t get that.

  2. rugger9 says:

    That the FBI is trying as hard as they are to ignore the herd of evidence elephants in the room tells me that they have run out of patsy candidates and must find any way to pin this on Ivins. The only way that narrative works is that Ivins had to be able to teleport himself to NJ, make weaponized anthrax by himself without detection in a facility not designed or capable for the necessary work [a skill that we called in the Navy SFM, for “sheer f___g magic”] since no one logged any out-of-place activity nor reported it to supervisors in an extremely sensitive facility.

    Why anyone would give the FBI any credibility in this investigation after they’ve blown their third strike [after Iraq and Hatfill] would probably involve a serious amount of payoff money and/or protection for very powerful MOTUs, involving a very short list. If the scientific community has issues with the FBI, it’s because the science was crappy, and the only reason to smear with the “critic” label in its various forms is a sad ad hominem attempt to deflect attention away from the facts. Willman doesn’t provide any rational explanation other than “the FBI said it, so it must be true by definition”. Now, who said “we make our own realities”? IIRC, it was Darth Cheney, my vote for Suspect #1.

  3. Ken Muldrew says:

    Jim, do you know why someone would want to put polydimethylsiloxane (PDMS) on the spore coat of antrax? Is it to create a hydrophobic coat that can more readily attach to surfactants in the lung? To chelate hemoglobin? Or is there some reason that is completely unrelated to increasing the virulence?

    From the files of odd coincidences, I put PDMS on my windshield just yesterday, having struggled to scrape a particularly hard frost off of it in the morning.

  4. Jim White says:

    @emptywheel: In the picture, the lower panel is a conventional electron micrograph that shows the overall structure of an anthrax spore from the attack material. The upper panel is the same view of the same spore, but seen under conditions where specific chemical elements can be identified. In this one, silicon glows green, and it is found only on the outside edge of the spore coat, and not at all on the outer part, the exosporium. When Michael used this same technology to look at other anthrax spores that he treated with silica nanoparticles (not shown in the figure), he only saw the green glow on the exosporium and not on the spore coat. So Michael proved that one method for treating the spores gave a different result than he saw with samples from the actual attack material. But he did not treat spores with the process Hugh-Jones and friends described, and the quote from him that Willman runs makes it look like he doesn’t accept their reasoning from the chemistry of the treatment that this treatment could give a result similar to that seen in the attack material. It is important to point out that this experiment has not yet been carried out as far as I know, so the likely outcome of tin-catalyzed silicon polymerization treatment of anthrax spores remains a hypothesis right now. An experiment to test this hypothesis is one that the FBI could have either Dugway or Batelle carry out, but that seems extremely unlikely.

  5. Jim White says:

    @Ken Muldrew: The belief is that spores treated in this way would aerosolize quite readily, matching some of the reported observations that the attack material in the Leahy/Daschle samples did suspend very easily in air.

    And yes, treatment of windshields and laboratory glassware with a coating of PDMS is a time-honored process for making them repel water. Just pouring the solution of precursor molecules you put onto your windshield to coat it with PDMS onto spores wouldn’t be expected work if the spores were still suspended in water, since the precursor molecules would react with the water before they got to the spores. Instead, as described in the paper, the spores would need to be dried and then suspended in an organic solvent prior to mixing with the appropriate precursors and tin catalysts.

  6. rosalind says:

    ah, so the LA Times has decided to double-down on David Willman’s anthrax narrative. i would’ve thought they’d have kept their heads low and consigned their man Willman’s – “Ivins did it cause look at him he’s soooo weird. And sororities” – to the dustbins of their crappy search engine.

  7. What Constitution? says:

    Thank you for this analysis, Mr. White. I may live a very long time before I ever come across the phrase “a tin-catalyzed polymerization of silicon-containing precursor molecules” block-quoted in something I read — and it will be longer still before I understand it, I’m sure. But I also therefore appreciate the introduction, in comments, to the concept of “SFM”, that works for me. And in the meantime, I’m still waiting for Brian Ross to identify the four “confidential government sources” who falsely asserted that the anthrax contained bentonite, indicative of Iraqi origin so that we can learn what the thought process behind such leaks actually was.

  8. rugger9 says:

    @What Constitution?:
    You won’t hear it from Ross, he’s been a tool as long as he’s been on the air, even back in the sports reporting where he specialized in gotcha “journalism”. You won’t hear it from the WH either, even with the change in administrations. However, since Darth is still playing the “tough guy” role, he might let some stuff slip in an interview, that report had his stench on it.

  9. pdaly says:

    Great post, Jim.

    The terms ‘spore coat’ and ‘exosporium’ are a bit confusing to the uninitiated as “coats” are usually worn on the outside–so it is difficult thinking about two outsides.

    I think a helpful analogy would be a human subject standing naked inside a clear plastic bag but receiving a spray on tan from an external source despite the plastic barrier protecting the human skin.

    The chemical spray coats the person’s skin (spore coat) and not the external plastic (exosporium). In spite of penetrating the plastic, the tanning chemical does not coat the exterior surface of the bag, thanks to nanoparticles’ ability to slip through the bag’s defenses and the helpful hand of the tin catalyst.

  10. pdaly says:


    Does this tin catalyst polymerization theory obviate the need for fine milling to produce the floating spores of weaponized anthrax, or is the milling an additional step Ivins supposedly would have had to take in an already less than adequate time frame?

  11. Jim White says:

    @pdaly: Interesting analogy, thanks. It works pretty well, but I would add that in the spore case, the silica nanoparticles are too large to penetrate the outer layer; it is the even smaller silicon monomer molecules that slip through the pores.

  12. Jim White says:

    @pdaly: I think that the assumption is that if the spore preparation is clean enough before it is dried and subjected to the polymerization treatment, the spores wouldn’t stick to one another and so they wouldn’t need milling. In the paper cited, this is believed to be the case for the letters sent to Daschle and Leahy. The New York Post material, on the other hand, has additional material besides anthrax spores (including B. subtilis spores and possibly non-spore cells of both bacterial species), and the paper explains how having this material present could lead to the observed clumpier nature of this material.

  13. rg says:

    @Jim White: Jim, thanks for this explication. Much is clear, but I’m still lost as to some things. One is in regard to this statement from @14: “having this [additional] material present could lead to the observed clumpier nature of this material”. I’ve been operating under the assumption that there were two “grades” of anthrax mailed out, one being the “weaponized” (I don’t care for this term-even spellcheck doubts it). Should I take from the above statement, that but for the additional material, all the mailed material were of the same “grade”.

  14. Jim White says:

    @rg: That one’s a tough call. There is evidence that the spores in the Post material did have as much silicon inside them as the spores sent to the Senators. However, there was a LOT more silicon in the Post material that wasn’t inside spores; it was associated with the extra material that was present. The most likely story then, is that the Post letter had “weaponized” spores, but the spore prep was not clean enough to result in material of the quality seen later.

  15. pdaly says:

    Since every day it increasingly looks like Ivins is just another fall guy –after Hussein (WH/BushCo inspired) and then Hatfill (FBI)– and increasingly impossible for Ivins to be a lone wolf, does anyone theorize why, if this was a false flag op, the Ames strain was used instead of a non-US controlled strain of anthrax?

    DNA testing of “morphs” (of anthrax colony mutations with fermentation) was not yet even in its infancy in 2001 so maybe this was an unknowable and obviously unwelcome development for the (USG?/?Bioport?/Batelle?/other?) perpetrators.

    The FBI’s insistence on the Lone Wolf theory has never been adequately explained. Jim White’s posts and emptywheel’s timelines cast doubt on Ivins’ ability to do any of this alone or to do all of this at his work lab.

    The FBI ignores timeline constraints when it comes to Ivins’ ability to lone-wolf-drive to NJ and back but invokes timeline constraints for lone wolf would-be perpetrators at Batelle, etc, ‘because they work in a buddy system’ and ‘because the airflights from Ohio to New Jersey and back to Ohio would not fit any one worker’s schedule for that time period’.

    BTW, Nova is encouraging high school students to learn more about the anthrax letters. Hope they are reading and FDL

  16. rg says:

    @Jim White: Jim, Thanks for the prompt repose. I’m hoping this isn’t keeping you up late; it’s nowhere near 10 PM where I am. And while at it, I hope your recovery is going well.

    Now, we can start using the term “siliconized” instead of “weaponized”. Your response indicates to me that its more plausible (than what I had been thinking) that the same person made both “grades” of anthrax. Still though, wouldn’t one who had the lighter anthrax also have the purer, thus having no need to experiment with “cleaning”?

  17. Jim White says:

    @rg: The belief is that the dirtier batch was produced earlier and then the cleaner batch was produced after some improvements to the processing of the spores after culture.

  18. Jim White says:

    @pdaly: No envelope was recovered from Florida. Some environmental isolates of anthrax (and B. subtilis) were recovered from the building later, but, inexplicably, the FBI chose not to carry out genetic analyses. This is yet another very disturbing decision they made.

  19. pdaly says:


    Maybe we should use the term “siliconTINized”, since a silicon coating on the spore coat can occur naturally–but only a thin layer of silicon will deposit this way.

    By comparison, the Amerithrax anthrax spores, have much larger coats of silicon on their spore coats than occurs naturually and even larger spore coats than can be achieved with the same Ames strain grown in a silicon rich broth. As Jim White mentions, the tin content is a telltale sign that the silicon/tin spore coats of the Amerithrax anthrax were added after the anthrax was dried, because tin is otherwise deadly to anthrax growing during a fermentation stage.

    Seems we have come full circle. Early in the Amerithrax investigation, the government was at pains to state the Amerithrax powder was NOT weaponized with post production silicon (no silicon on the exosporium), because to say as much would implicate government agencies.

    The government reassured the public that the silicon was only seen on the anthrax spore coat implying that this anthrax was grown naturally but just finely so…

    Now the silicon-tin spore coat more likely implicates government agencies, again.

  20. al75 says:

    My decades of study of the JFK assasination, and my own experience of the forensic process in court. have made me wary of non-experts attempting to interpret specialized technical data.

    A few non-expert questions:

    Was the tin-silica “weaponization” process a known technique prior to 2001?

    If it was, why wasn’t it considered by the FBI and various analysts until recently?

    If not – when did this technique become widely known? Has anyone ever used it, or is it merely a post-hoc theory?

    If this technique has been used, who used it? Do any published scientific works refer to it? If it is unpublished, secret weapons work – who had access to information about this technique?

    It seems to me that if a sophisticated weapons technique exists, sufficient familiarity with it could be traced to a fairly small number of people/places.

  21. Jim White says:

    @al75: The silicon-tin process for coating materials in general was known in 2001, but whether the government knew of its use for weaponization of biological agents or was capable of it is not known, because such work would have been classified and also would have been in violation of bioweapons treaties. Some research on weaponization technology in general is believed to have been carried out, though, under cover of a claim that it was being for purposes of defending against other actors making such weapons. Whether that work specifically involved silicon-tin is not known.

  22. rg says:

    @al75: With regard to the question of how widely known was the technology of “lightning” (making lighter), there is anecdotal info (data?) to consider. Somewhere in the mass of writing on the Amerithrax case, there was an episode told by a person who worked at the Ft Detrick lab where Ivins worked. She described how a sample of Anthrax, which had been developed at another lab, somehow was available for examination at USAMRIID. Apparently there was much oohing and ahhing over the miraculous way the spores just floated in the air, and wonder at how on earth such had been achieved. I got the impression that Ivins appeared just as amazed as the others. I believe this was after the mail attacks.

    As I have alluded in comments to Jim, the term “weaponization” seems to be a nontechnical term with pejorative connotations, and as Jim just mentioned, in violation of treaties to control biological warfare. The development of Anthrax with increased dispersal characteristics would constitute a secret, and would not be widely known.

  23. pdaly says:

    FYI: Stuart Jacobson posted the following on Oct 12, 2011 at

    regarding his group’s paper that was to be published:

    Our paper highlights the extraordinary presence of tin not merely by its qualitative presence but by the quantitative presence.

    The New York Post powder contained 0.65% tin, which is equivalent to 6500 parts per million (ppm). Tin was present in the highest quantity of any metal with the exception of calcium – but calcium has well- understood biochemistry and is known to be required in the biology of anthrax spores. Calcium, as well as many other metals, is deliberately added to spore preparations before spore growth occurs. A typical set of precursors in the preparation of bacillus spores is yeast extract, NH4SO4, MgSO4·7H2O, MnSO4·H2O, ZnSO4·7H2O, CuSO4·5H2O, FeSO4·7H2O, CaCl2·2H2O, K2HPO4 and glucose. Thus it should come as no surprise to find magnesium, manganese, zinc, copper, iron, calcium and potassium as major components in spores – and they generally are found as major components, i.e., >100ppm. Other elements like chromium are often found as contaminants in chemicals close to them in the periodic table, such as manganese (for chromium). These might be expected to be found in quantities of 1-10ppm.

    For tin to be found at concentrations greater than iron, potassium, magnesium, manganese in the New York Post powder is extraordinary by any measure. The idea that tin is an accidental contaminant in this case when it’s concentration is higher than other metals that were deliberately added is not a sound hypothesis.

    We have proposed a set of circumstances involving microencapsulation of the spore coat to explain what has been observed in the attack spores. Now this hypothesis needs to be tested in a competent laboratory to see if the proposed chemistry is correct and whether the percentages of silicon and tin in the test microencapsulated spores match what was measured in the attack spores

  24. pdaly says:

    From the same link, author Barbara Hatch Rosenberg‘s comment 4 days ago:

    Briefly condensed, the story on additives in our still-to-be published paper, referred to in the foregoing article, goes like this:
    The attack anthrax spores have been found to contain silicon, oxygen and tin on the surface of their spore coats, unlike normal anthrax spores. Because tin is toxic, and is present in substantial amount, it must have been added after the spores were grown. Although silicon occasionally occurs naturally on the spore coats of anthrax spores, the amount of silicon on the attack spore coats was much greater than that on other spores, even those with the highest amounts, which implies that silicon may also have been an additive. The elements present on the spore coats suggest that the spores might have been silicone-coated. Tin is a catalyst for cross-linking silicone polymers. The small, silicon and oxygen-containing molecules (silanes) that react to form a polymerized coating can pass through the outer membrane of the spore (the exosporium) to reach the spore coat, where moisture is present to complete the reaction. Unlike the anthrax powders in the Senate letters, the powders in the earlier attack letters were impure and contained cellular debris, which could also react with the additives; as a result, the bulk NY Post letter powder would have a much higher silicon and tin content than the bulk Senate powders, in accord with the observations. Microencapsulation with silicone has been used to confer high stability on biologicals, protecting them from environmental hazards–not a property relevant to their use in the letter attacks, but of interest to biodefense. (The assumption that additives must necessarily be related to dispersibility does not necessarily apply in the case of anthrax spores, which are surrounded by the exosporium.) If the attack spores were microencapsulated, they were probably made for some purpose other than letter attacks. For example, DARPA’s project on CBW detection intended to look at microencapsulated pathogens in 2001, as stated in the June, 2001 DOD FY 2002 Amended Budget Submission for DARPA.

  25. Kevin Dugan says:

    I’ve been following this story off and on for years but don’t recall whether the noted Soviet Bioweapons defector Ken Alibek has been involved, either as a potential suspect or as a consultant.

    Does anybody know whether that trail has been followed – ie Alibek, his associates, his US Gov. bioweapons defense contracts, his potential access to/knowledge of equipment/facilities capable of producing the tin-siliconized anthrax or his knowledge of who probable suspects might be?

  26. Jim White says:

    @Kevin Dugan: Alibek figures prominently in Richard Preston’s “Demon in the Freezer” which covers a lot of the investigation. He serves as an info source for Preston in the book.

    Also for rg @26: The episode of the floating spores is described in this same book and relates to material from one of the Senate letters being investigated at USAMRIID.

  27. al75 says:

    wow – thanks for all the excellent, informative posts. I will follow the various links and then bore my wife by pretending to be a bioweapons expert.

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