MIAMI , Florida ( CNN ) -- A judge sentenced a rogue FBI agent to 40 years in prison on Thursday for the 1982 mob-related killing of a witness who was about to testify against Boston mob members , court officials said . Former FBI agent John Connolly `` crossed over to the dark side , '' a judge said . Disgraced ex-FBI agent John Connolly Jr. `` crossed over to the dark side , '' said Miami-Dade Circuit Court Judge Stanford Blake . The sentence will run consecutively to a 10-year racketeering sentence . Connolly , 68 , was convicted in November of second-degree murder in the death of businessman John Callahan , an executive with World Jai-Alai . Callahan 's bullet-riddled body was found in the trunk of a Cadillac parked at Miami International Airport . Connolly 's fall from celebrated mob-buster to paid gangland flunky captivated a South Florida courtroom for weeks . In testimony at his sentencing hearing last month , he denied having any role in Callahan 's death . `` It 's heartbreaking to hear what happened to your father and to your husband , '' he told members of Callahan 's family . `` My heart is broken when I hear what you say . '' He explained , in the face of vigorous cross-examination , that rubbing elbows with killers and gangsters and winning their confidence was part of his job . His attorney argued that Connolly did what the FBI wanted him to do , and now was being held responsible . Connolly did not testify at his trial . Prosecutors had asked that Connolly be given a life sentence , saying the 30-year minimum was not enough because Connolly abused his badge . In a Boston Globe interview published last month , however , Connolly vigorously denied being a corrupt agent . `` I did not commit these crimes I was charged with , '' Connolly told the newspaper . `` I never sold my badge . I never took anybody 's money . I never caused anybody to be hurt , at least not knowingly , and I never would . '' During his two-month trial , jurors heard that Connolly told his mob connections that Callahan , 45 , was a potential witness against them , setting him up for the gangland-style slaying . According to testimony , Connolly was absorbed by the very gangsters he was supposed to be targeting -- members of South Boston 's notorious Winter Hill gang . His story was said to be the inspiration for the character played by Matt Damon in the 2006 Martin Scorsese movie , `` The Departed . '' Connolly 's tale was closely followed in New England , where he grew up in Boston 's `` Southie '' neighborhood , the same area long dominated by the Winter Hill gang and its notorious leader , James `` Whitey '' Bulger . Sought in 19 slayings , Bulger is the FBI 's second most-wanted fugitive . During the first two decades of his FBI career , Connolly won kudos in the bureau 's Boston office , cultivating informants against New England mobsters . Prosecutors said Connolly was corrupted by his two highest-ranking snitches : Bulger and Stephen `` The Rifleman '' Flemmi . Connolly retired from the FBI in 1990 and later was indicted on federal racketeering and other charges stemming from his long relationship with Bulger and Flemmi . He was convicted of racketeering in 2002 and was serving a 10-year federal prison sentence when he was indicted in 2005 in the Callahan slaying . During testimony , jurors heard that Connolly was on the mob payroll , collecting $ 235,000 from Bulger and Flemmi while shielding his mob pals from prosecution and leaking the identities of informants . The prosecution 's star witnesses at the Miami trial were Flemmi , who is now in prison , and mob hit man John Martorano , who has admitted to 20 murders , served 12 years in prison and is now free . Callahan , who often socialized with gangsters , had asked the gang to execute Oklahoma businessman Roger Wheeler over a business dispute , according to testimony . Martorano killed Wheeler in 1981 on a golf course , shooting him once between the eyes , prosecutors said . After Connolly told Bulger and Flemmi that Callahan was going to implicate them in the slaying , Martorano was sent to do away with Callahan , prosecutors said . But one star witness did not testify -- the former FBI agent who inspired the 1997 film `` Donnie Brasco . '' He refused to take the stand after the judge denied his request to testify anonymously .
NEW : Ex-FBI agent `` crossed over to the dark side , '' judge says . Sentence will run consecutively to previous 10-year racketeering sentence . John Connolly Jr. has denied role in 1982 mob hit . He was convicted of second-degree murder in slaying of gambling exec .