The hunt was maddening. All summer and into the fall, a bunch of FBI irregulars called the special surveillance group--the "G's" in bureau lingo--shadowed Stanislav Gusev when he angled for his favorite parking spot near the State Department, then settled onto a well-worn bench. Whenever Gusev, 54, a technical specialist for the Russian intelligence service, fiddled with something in his pocket, the G's state-of-the-art radio-signal detector would come to life, indicating that a faint low-frequency transmission was emanating from a bug somewhere in the gray State offices.
But where? While the G's, dressed down like tourists, students and street people, kept...