Complex sentence: Sheriff Fred Diamond thought the men seemed sincere, and genuinely frightened and thought Parker was especially disturbed, but but he harbored some doubt about the fantastic story, due in part to Hickson's admitted whiskey consumption.
Split into simpler sentences: Sheriff Fred Diamond thought the men seemed sincere and genuinely frightened and he thought Parker was especially disturbed. Diamond harbored some doubt about the fantastic story, however, due in part to Hickson's admitted whiskey consumption.