The Berkeley Radiation Laboratory espionage investigation transformed a secretly recorded, ambiguous conversation into a widening campaign of suspicion against young physicists associated with J. Robert Oppenheimer. Its central evidence came from an illegal FBI microphone in Steve Nelson’s home; its consequences fell not only on the unidentified speaker eventually identified as Joseph Weinberg, but also on Giovanni Rossi Lomanitz, David Bohm, and Max Friedman. The investigation exposed how wartime counterintelligence could turn political association and circumstantial connections into professional punishment, while leaving the underlying allegations unresolved.