That question has confounded investigators throughout the probe into the 2001 anthrax letter attacks, which the U.S. government formally concluded in February. Scientists inside and outside the government say there is clear evidence that the high levels of silicon found in the anthrax came not from anything added to "weaponize" the anthrax spores—as researchers had suggested early in the probe—but from the culture in which the spores were grown. That evidence may have settled the issue of whether the anthrax was weaponized, at least for scientists familiar with the case. But it raises a different question: Why did the mailed anthrax have such a high proportion of spores with a silicon signature in comparison to most other anthrax samples?
... The FBI's scientific case against Ivins rests on DNA tests showing that the mailed anthrax came from a flask under Ivins's control at the U.S. Army Medical Research Institute of Infectious Diseases in Frederick, Maryland. Investigators also had the attack material chemically analyzed, first at the Armed Forces Institute of Pathology (AFIP) in Washington, D.C., within weeks of the attack. Examining the spores under a scanning electron microscope, AFIP scientists detected silicon and oxygen and concluded that the spores had been coated with silica to make them float easily, enhancing their power to kill.
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 the New 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.
The unusually high percentage of silicon-bearing spores in the attack material "is a bit of a strange thing," says Michael. "We have no way of knowing how they were really grown." An anthrax researcher who did not wish to be named calls it "awfully weird" and "a particularly inconvenient exception" because it leaves a gap in the case. However, neither scientist thinks the anomaly casts doubt on the broader investigation...