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The White House has said that the U.S. intelligence community has concluded “with varying degrees of confidence” that Syria’s regime used the nerve gas sarin. As that statement suggests, such judgments usually involve shades of gray. In this case, there are still many unknowns: how was the evidence obtained and under what conditions the chemicals were used. Larry Abramson talks to Robert Siegel about what the U.S. knows and what it does not.