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Meanwhile, Meselson and others believe there might be a pattern in the attacks so far. Focusing on the three letters postmarked Trenton, N.J., they note that the first two, mailed Sept. 18, contained a relatively crude form of anthrax and caused mostly skin infections. The third, mailed to Daschle on Oct. 9, carried more potent spores; infections plausibly caused by that letter were mostly inhaled. Meselson has proposed that the initial mailings could represent the first two tentative steps in a diabolical experiment that's not over yet.
If so, a third and potentially far more lethal step could still be in the works. Even experienced bioweapons experts were surprised to learn how much weapons-grade anthrax showed up in Daschle's office; they don't even want to guess how many people might have been infected if it had got into the building's ventilation system. For now, authorities are keeping their eyes on crop dusters and skyscraper air vents, but if someone managed to puff a good-size cloud of anthrax into a large, enclosed area--a basketball arena, say, or a city subway system--the death toll could be in the hundreds or even thousands.
--Reported by Andrea Dorfman, Stephen Handelman and Alice Park/New York City and Elaine Shannon and Andrew Goldstein/Washington