In 1967, following a three-year investigation, Fonzi co-authored a Philadelphia magazine article exposing the activities of Harry Karafin, an award-winning American investigative journalist associated with The Philadelphia Inquirer who sought and accepted payment from potential reporting subjects in order to avoid negative coverage.[4][5] Karafin was fired and then convicted on 40 counts of blackmail and corrupt solicitation.[6]
In 1975, he was hired by Senator Richard Schweiker as a researcher for the Church Committee into the activities of U.S. intelligence agencies, and in 1977 he was hired as a researcher for the House of Representatives Select Committee on Assassinations (HSCA). According to The New York Times, Fonzi was recruited as an investigator for the HSCA "mainly on the strength of scathing magazine critiques he had written about the Warren Commission and its conclusion that Lee Harvey Oswald had acted alone in killing the president in Dallas on Nov. 22, 1963."[3]Gerald Posner wrote: "Fonzi seems an unusual choice for an inquiry that claimed to be impartial, as he was a committed believer in a conspiracy, having written his first article critical of the Warren Commission in 1966."[9][nb 1]
In his work for the HSCA, Fonzi focused on the role of Cuban exile groups and the links those groups had with the Central Intelligence Agency (CIA) and the Mafia. He obtained testimony from Antonio Veciana that the latter once saw his CIA contact, whom Fonzi established was David Atlee Phillips, conferring with Lee Harvey Oswald.[1] In the course of Fonzi’s research, he attempted to interview Oswald's friend George de Mohrenschildt on March 31, 1977; hours later, de Mohrenschildt was dead, an apparent suicide.[11]
In 1980, Fonzi published an article in The Washingtonian on the JFK assassination. The article aroused enough interest from the CIA for it to investigate whether Fonzi had breached his 1978 non-disclosure agreement with the CIA, which he had signed in order to gain access to classified files (it concluded that Fonzi had not).[12] The article later formed the basis for his 1993 book, The Last Investigation.[3] In 2012, The New York Times said of The Last Investigation that "historians and researchers consider Mr. Fonzi's book among the best of the roughly 600 published on the Kennedy assassination, and credit him with raising doubts about the government’s willingness to share everything it knew."[3]