TODAY | March 01, 2014
>>> this next story seems like something out of the movies, but it was all too real. a rookie detective in rhode island going deep under cover to bring down the new england mafia. it really happened, and two decades later, he can finally tell his story. here's andrea canning.
>> reporter: from whitey bulger to the mob wives --
>> don't, don't.
>> did i get her in the face?
>> reporter: even after "the sopranos" --
>> johnny's in jail.
>> reporter: -- america's fascination with the mob is alive and well . for steven o'donnell, growing up in a blue-collar neighborhood in providence, rhode island , organized crime was a fact of life, and the patriarch of mob family was top dog.
>> i was fascinated with organized crime .
>> reporter: fascinated but determined to stay on the right side of the law. o'donnell started off as a maximum security prison guard, learning the language of the mobsters.
>> without question, we work in the prison set me up for some type of success, because i could walk the walk, talk the talk.
>> reporter: on one fateful day in 1994 , on the steps of the providence courthouse, o'donnell, now an undercover cop, found himself face to face with harold tillinggast, a mob hit man out on parole. you seized the moment.
>> i was coming to get a warrant signed unrelated. i saw him and i had to get rid of the warrants i had and then just talked to him.
>> reporter: talked not like a cop but like a guy from the hood, convincing tillinggast they had done time together. he just bought everything, hook, line and sinker?
>> i convinced him i was an inmate. i knew what the cell block meant, knew what he did for work.
>> reporter: and so began the big lie . o'donnell was foley now, steve foley , living someone else's life. he was now all in.
>> i was a bookmaker for them, so to speak, i was a drug dealer . they thought i was buying large amounts of cocaine, large amounts of heroin and large amounts of pot, marijuana.
>> reporter: six years in the shadows moving with mobsters, building a case, harold tillinggast was one thing, but his brother, jerry, was the big fish , running his
brac@s from prison. at one point, foley protected him from the wiretaps in prison.
>> from jerry's perspective, he basically took full ownership that steve foley , which is me, he's my guy, he pays me, and if they have a problem with that, have them some see me.
>> reporter: armed with police testimony, along with the tapes, the photos and the documents, the attorney general went after jerry and harold and other members of their mob family.
>> he caught my brother sleeping.
>> reporter: in a twist of irony, even jerry tillinghast had to respect what o'donnell pulled off.
>> if the shoe's on the other foot, would i do the same thing? probably, you know?
>> reporter: that same year, foley , now back to steven o'donnell, was honored by his peers and even the president. today, a picture hangs as a reminder of those six years under cover in the office of colonel steven o'donnell, now the head of the entire rhode island state police . for "today," andrea canning, nbc news, providence, rhode island .