Transcript for Andrew McCabe on President Trump's relationship with Russia

A former FBI deputy direct to Andrew McCain. Really ruined presidents' day for trot. Off. Who has been ranting about the shocking claims about him in the new book the threat. How the FBI protects America in the age of terror and trying to please welcome Andrew McCain. Okay. Good. It took over as acting FBI director F to your boss James call me was fired by trump. And almost immediately we've now learned. You opened investigations into whether trump obstructed justice and whether he was acting as an agent for Russia. Did you believe he was actually an agent for Russia. We believe that could be possible and that's why we open this came out possible. Well you know joy the FBI doesn't open cases because we like a person or we don't like a person we don't open cases because we are. Now political in one direction and other human cases because we look at the information we have in our hands at the time. If that information. Gives us a credible ticket will basis to believe that a threat to national security exists or that a federal crime may have been committed. It is our obligation under those circumstances to open a case that's the determination we went through and decided to open a case on the present. Quickest we an hour who are we talking about how many what what what goes into making that decision so called or so that was the recommendation came to me from the investigative team that was responsible for. The investigation of Russian interference into the 2016 election. It was something that we considered very closely I further disgusted with FBI attorneys General Counsel and of course I had several conversations. With rod Rosen Stein about it. Following that process ride a night together briefed congressional leadership about the steps we've taken. But to please add devil's advocate on this it looks like it was an attempt to lash out at trump who had just fired her plots. Do you actually believe that Russians ordered trump to fire homey. Well we don't know. And we certainly didn't know that at the time. So Megan I think it's important to realize that the things that were causing us to believe that a threat to national security might exist. Had been building up throughout the investigation we've been conducting through the fall. It was the way the president had been interacting with us the way that he had talked about the case publicly. Tried that kind of undermine our efforts made it clear that he wasn't happy about what we were doing the fact that he had asked Jim call me. To shut down the investigation into Mike Flynn which was an important part of that investigation. And then of course the fact that he fired the director when he didn't do those things that the that the president asked so he put all those circumstances together. And then of course add to it the president's own public statements indicating that he's thinking about Russia when he fired the director of the FBI. We felt it was. Our obligation to act in those circumstances was undeniable. We ask you this director yet a lot of meetings with. Deb Reza Stein during this time and I more than one occasion he offered you say seriously he said it was a joke. To Wear a wire into the White House. You say he also floated the idea of invoking the 25 amendment. To remove the president and speculated about which cabinet officials would be on board. So what goes through your mind when you hear the number two at the Justice Department but trump appointee. Basically saying that the president is unfit. To be the president of the United States. Sony was incredible I mean you worked in the department you understand how far we he'd been pushed to even. To even how the conversation that included those statements. And overstated because it do you think about the commerce this. These comments about the 25 amendment and about wearing a wire have been a little bit distracting and its pull people away from the main themes here. But nevertheless we were all operating under incredible stress we were all grappling with this idea that we have a president. We think may have committed obstruction of justice who we think may in fact be a threat to national security. What do we do in this circumstance to even see that. In those conditions that is incredibly stressful times. Rod Rosen Stein who had been in the position only for a few weeks and this kind of figuring these things out it seemed as he went along. Tom does not outrageous that he would make comments off the top of his head that would things that we of course did not follow up on. That the DOJ did respond saying Rosen Stein never authorize recording the president and was not in a position to consider invoking the 25 amendment. In my mind that's not exactly a denied. Well it's not. It is true. That the deputy attorney general never authorize. Conducting a recording of the president. That is because we never asked for that authorization this was a comment should do rod should get half. I don't believe so now now I don't believe so it's something that we talked about but never seriously considered it seemed like. Such a overwhelmingly in risky and unnecessary thing to do are you can't. And I ask it was a week when you actually is that same small group of people that I was working with that the FBI investigative team. And Mike counsel at the filling those. BI film but are still big things we talk about rep Rosa shine bring up the 25 amendment talking about wearing a wire it wasn't you didn't write that the book which surprised me because. That's one of the biggest pieces of what we've experienced what the talking about the past few years but. Trumpets and the editors. Anything's from the beginning. It's been sort of your mission to get him out of the White House he tweeted over the weekend saying. That your story is deranged T said that UN rot Rosa Steinberg quote planning a very illegal acting got caught even senator Lindsey Graham. Head of the senate judiciary committee's cities launch an investigation that this is a coup. Was this a coup from the beginning to that's not just Lindsey Graham you have as you know many people that are questioning what happened during that time. Source the fact that the president uses the word to or uses language is as tweets that aren't true that are blatantly false is not should not be surprising to us. The president has been lying about me in my family for the last two years so when I see tweets like this. It's kind of sad to say that we've become kind of used to it by now. The fact is. And it's not in my book for exactly this reason I made the decision not to include these comments in the book because they were exactly that it never went beyond. The realm of the brief off handed comment by the deputy attorney general to me I'm not aware of any steps that were taken to execute this I'm not aware of any meetings that were pulled together to discuss it. There was no further discussion that I'm aware of and I was afraid that it would distract. From the very important things and I'm trying to talk. And it went went went from tweets against you. Don't get fat honey get happy is the best thing that can happen play a book sales.

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