Complex sentence: Nosenko claimed that he could provide important negative information about the assassination of President John F. Kennedy, affirming that he had personally handled a review of the case of Lee Harvey Oswald, who had lived in the Soviet Union prior to the Kennedy assassination.
Split into simpler sentences: Nosenko claimed that he could provide important negative information about the assassination of President John F. Kennedy, affirming that he personally handled the case of Lee Harvey Oswald. Oswald lived in the Soviet Union prior to the Kennedy assassination.