LIKE MANY GREAT SPY STORIES, this one begins with a brief, mundane scene whose significance only becomes apparent later on. Around lunchtime on February 1, 2000, a man dropped a piece of paper near a table in a Middle Eastern restaurant outside Los Angeles and paused long enough to strike up a conversation with two Arabic-speaking men dining nearby. It would take FBI agents nearly 20 years to understand the full meaning of that small event. The man who dropped the piece of paper was Omar al-Bayoumi, a Saudi intelligence asset, recently declassified FBI documents show. And the two Arabic-speaking...