Complex sentence: The chief prosecutor in the case was Robert B. Fiske, then the United States Attorney for the Southern District of New York, who was assisted by two younger Assistant United States Attorneys, Tom Sear and Bob Mazur.
Split into simpler sentences: The chief prosecutor in that case was Robert Fiske, then the United States Attorney for the Southern District of New York. He was assisted by two younger Assistant United States Attorneys, Tom Sear and Bob Mazur.