According to the FBI lab, 1.4% of the powder in the Leahy letter was silicon. "This is a shockingly high proportion," explained Stuart Jacobson, an expert in small particle chemistry. "It is a number one would expect from the deliberate weaponization of anthrax, but not from any conceivable accidental contamination."

Edward Jay Epstein: The Anthrax Attacks Remain Unsolved - WSJ.com