Transcript for James Comey Part 1: Telling Trump about the 'dossier'

or George Stephano. Reporteomey. A polarizing man, for a polarized time. Llary Clinton sahat he cost her the white house. Do tt called him an untruthful slimeball. Now, he is sharing his story ou at the crossroa of the most controversial election in modern history. You say it's a dangerous in our try. I think it is. And I chose those words carefully. I was worried when I chose the word "Er first. I thought, is that an overstatement? And I don't think it is. Reporter: Why not? I worry that the norms at the centthis country, an fight as Americans about guns, taxes or immigrn. We always have, but what we have in common is a set of norms.momportantlythe truth. And if wse that, if we lose tethering our leadeo that what are we? Reporter: I've talked to a lot of people who've watched you for a long T some of them come out with this storyline on James Comey. "Here was Y loved Bein FBI director. He got fired. He'sry." I get why pe would thk that. That's just not right, though. Repr: Welcomeo election night 2016. What a C campaign this H been. In his new book "A higher loyalty," Comey describes the fateful decisions around the 2016 presintcampaign. Where were you as the returns were coming? Ihink I was home that night. Reporter: And test states are in. So let's right to it. U didn't vote? No. Reporter: Why not? Iirector ofhe FBI. I'm Ying to beutside of politics. That I shouldn't be choosingbethe candides. Reporter: Look at that, george.donald trump is back in the lead in the state onnsylvania. I was surprised. Reporter: If he wins Pennsylvania, it is preselect trump. All of us were operating world where the polls were showing that Donald Trump had Chan R he is now going to be the 45th president the Ed states. The motuupset in American political histo elect Donald A whole lot of me was thinking, oh, my god.d we have me role inhis? Weme impact on the election? Reporte does UT tell you? I don't know the answer. And in a way, I care about the answ and in a way, it doesn't matter at all. I really wasn't making decisions sed on political fnes. Eporter: Oa cold winter day in January, Comey heads to trump tower to meet with the presidenect for the fi time. Were you nervous? Yes. Reportehat were you afraid of? W'm about to meet a person who doesn't know me, who's just been elected president of the United States. From my Wang him durg the campaign, could be volatile. And I'm about to talk to him about allegations that hs invo with prostutes in Moscow and that the Russians taped it and have leverage over Reporter: It was the first time you monald trump. What was ? He had imprvely coiffed hair. It looks to be all his. I confess, I stared at it pretty closely and my Rion was, it St take a lot of time in the ING. His tie was too long, as it always is.he looked hte up whichasre from tanning goggles. Eporter: C and a group of intelligence agency heads were to brief the president-elect about a disturbing report on Russian meng in the eltion. What was their reaction? Presidelect trump's F question was to conftht Hano impact on the election. Then the conversation, to my surprise, moved into a pr conversation a hhe trump team would position this and they could saout this. That's just not done. Reporter: You asaid you were struck by what they didn't ask. Vy much. No one, to my recollection, asked, "So, what's coming next from the Russians? How might we stop it? What's the future look like?" There was none of that. It was all, what can we say about what they did unit affects the election that we just had. Reporteou as this was happening, you had a ashback Toour early days as a prosecutor. I had a flash my days investigating the mafia, la cosa nostra. Reporter: Decades earlier, Comey learned la cosa nostra, "Our thing" in Italian, as a attorney prosecu the Gambino crime family headed by John gotti. In the mafia, a man is measu by the strenof his loyty. There's aistinction between a friend of yours and a friend of ours. I felt thifoke us all, and maybe this wasn't their intention, but it's the way it felt to make us all "Amica nostra."we're all part of messaging. We're all part of the effort. Boss is at the Hof the table and wegonna fig out gether how to do this. Reporter: How strs it for you to sit here and comp the prest to a mob boss? Very strange. And I don't do it liy. And I'm not trying to, by that, by the way, suggest president trump is out breaking legs and shaking down shopkes. But ind,talking about ishat leadership culture constantly comes back ten I thinouxperience with the trump administration. Repr:u think Yo shou have said something then? Maybe. I think the reason in't, I hope is obvious to folks, is about to stay behind to talk about alletiat the president being involved with prostitutes in mos and I thought, that's got to be my focus. Reporter: He was about to tell trump tteele ssier. Saous. Unverified. And so expveat Comey suggested eryone else leave the room, which they did. How graphic did you get? I think as graphic as I needed to be. I dinot go into the business about people peeing on E and he interrupted, started king about it. You know, "Do I look like a guy needs hookers?" I didn't ar that, ani just D on and explained, "Sir, I'm not saying that we credit th M not saying we believe it. We just GHT it Ver important that know." Rr: What was the look on his face? He was very defen and starting into aist of people who had accused him of touching them impropsexual ass how he hadn't done this, he hadn't done that hadn't do that Reporter: Did you tell him he Steele dossier Haen financed by his political opnents? No. 'T -- I don't thinused the term "Steele dossier," I just talked about additional material. Orter: But did he a right to knoat? That it'dn financedy his political opponents? I don't know the answer tot. Itasn't neceary for my goal, which was rt him that we had this informati Reporter: Did you Bel his Al I hy never thought ese words uld come out of my mouth, but I don't know whether the current president of the United States was with prostitutes peeing on each other in Moscow in 2013. It's possible, but I don't know. Ter: How weird was T briefing? Really weir I just wanted to get it done and get out of there. Reporter:ht after leave the ING, Comey began to take notes about trump's startling behavior, fethat the president-elect might later lie about what was said. I had, obviously, once about that earlier, having watchem on the campaign is someone who for whom the truth is not a high value. Reporter: The storyho James Comey came to have THA meeting withmp begins at a unassuming house in allendale, New Jersey, a suburb of New York City. How old are you here? I Thi about 6 in that picture and I was rocking things. It wasn't working in the60s either, but my mom cut our hair and that's the way she cut it. Eporter: Describe what life was like in this house. T N. My father always made it his business to be here for dinner and you'd havetalk about what happened to you during the day. Reporter: But in that house, an armed intruder broke in. Rrifying 16-year-old Comey and his younger brother. You say it C Y. How? It gave me a tremendous sense of urgency and the preciousness life. It also gave me great, great empathy for the victims of crime. Rtehe boy wourow up to enter the elite of law enement. Under George W. Bush. And, finally, director of the FBI. It was the job of a me, then came 2016 election, where his decisions have bee or changinthe course of history. Your critics say this is where your egoot the besof you. This was your original sin.

This transcript has been automatically generated and may not be 100% accurate.