LONG BEACH, N.Y.—Kenneth T. Robinson knew he should walk away. But in an interview with The Wall Street Journal, he says he just couldn't stop trafficking in insider-trading tips.

It was September 2009—a full 15 years after he'd first gotten involved in an alleged multimillion-dollar fraud that ranks among the longest-running insider-trading cases in U.S. history. The scheme was showing serious cracks: One of Mr. Robinson's two partners in the alleged crime, his close friend Garrett Bauer, had started throwing around big money—buying...