Vice Admiral Fujisaki was the Deputy Chief of Starfleet Intelligence, who died of "food poisoning" in 2374, ten days prior to Doctor Julian Bashir's original introduction to Luther Sloan and Section 31. (DS9: "Inquisition")
When he was recruiting Bashir for an elaborate conspiracy involving the Romulan Continuing Committee in 2375, Sloan informed Bashir that there were suspicions that Koval, head of the Romulan Tal Shiar, was involved in the death of Fujisaki, but no proof of this existed.
Later, Koval told another version of the conspiracy to the Continuing Committee, that Sloan in fact was the former protégé of the late Fujisaki. Sloan believed that Fujisaki was murdered by the Tal Shiar and he felt that the assassination of a Starfleet admiral was "stepping over the line." As a result, Sloan created Section 31 so that he could exact revenge against Koval without implicating Starfleet Intelligence.
The veracity of either account is extremely questionable, and it is known that Section 31 actually existed centuries prior to Fujisaki's death, and the circumstances of Fujisaki's death remain unclear. (DS9: "Inter Arma Enim Silent Leges")