I'm guessing, he probably investigated a bombing case (BO's MO ), and came upon the discovery of the BO. He started investigating a link, a famous actress at the time thought to be 'connected'. And as he tails her, he finds out a lot about her. A mistake slips, and the actress Vermouth finds out. She then decides to look up on him and finds out he is FBI, she decides to take him down without BO's knowledge so that no one would know of her mistake. After she does so, we know what happens, however little did she know that the spy had a daughter who could confirm his hidden documents regarding the existence of a large criminal corporate..