Michael Sassano is no ordinary stockbroker. The Oppenheimer & Co. ( OPY) broker might not rank as one of Wall Street's best-known wheeler-dealers, but in the shadowy world of mutual fund market-timing, he is a legend -- one who has caught the attention of investigators. (Oppenheimer & Co. is not related to OppenheimerFunds.) People who know Sassano, as well as sources familiar with the fast-expanding mutual fund investigation by federal and state regulators, used similarly strong language to describe the 33-year-old broker: "a big player," "a significant force," and a "major-league market-timer." Some of these people said that before the mutual fund scandal broke in September, Sassano had one of the biggest books of market-timing clients on Wall Street. They describe a man who carved out a niche of putting fast money together with funds that could be market-timed, the term for a legal but strongly discouraged strategy of making rapid-fire trades in mutual fund shares to capitalize on price discrepancies in different markets. In some cases, Sassano referred business to friends and associates who later became central players in the mutual fund scandal. All of this has piqued the interest of New York Attorney General Eliot Spitzer. TheStreet.com has learned that Spitzer's office has subpoenaed records from Oppenheimer to learn more about Sassano's activities, although prosecutors don't appear to be close to filing any charges against him. Spitzer's office wouldn't comment on its investigation. Officials at Oppenheimer & Co. did not return telephone calls. Sassano's name already has publicly surfaced in conjunction with the mutual fund investigation. He's identified, but not charged, in a civil fraud complaint filed last month by securities regulators in Massachusetts against a group of former Prudential Securities brokers. The brokers are accused of helping a group of hedge funds, some of which were referred to them by Sassano, to make rapid-fire trades in mutual fund shares. Additionally, Sassano, in the mid-1990s, worked at the same firm as three former brokers who have been either charged or implicated in the mutual fund scandal -- a coincidence that intrigues investigators.