MYRTLE BEACH, S.C. -- For months, a sophisticated hacker has been stealing the personal data of American corporate executives.

Hot on the hacker's trail is Joe Stewart. The former bass-guitarist-turned-cyber-sleuth stumbled onto the case in February. Since then, the 36-year-old Mr. Stewart has spent weeks in his office, in a nondescript building next to a half-abandoned strip mall here, virtually chasing the mysterious perpetrator across several continents. Mr. Stewart early on thought he had traced the scammer to China, then...