Transcript for Comey 'mildly nauseous' to think FBI affected 2016 election

Why was it necessary. To announce eleven days before a presidential election. If you were opening an investigation. On a new computer. Without any knowledge. Of what was in that computer. Why didn't you just do the investigation. As you would normally with no public announcement. Great question senator thank you. October 27. The investigative team that had finished the investigation in July focused and secretary Clinton's emails asked to meet with me. So I met with him that morning late morning in my conference room. And they laid out for me what they could see from the meta data on this Fella Anthony winners laptop that have been seized and on related case. What they could see from the meta data was that there were thousands. Of secretary Clinton's emails on that device. Including. What they thought might be the missing emails from her first three months as secretary of state. We never found any emails from her first three months she was using a Verizon Blackberry then and that's obviously a very important. Because if if there was evidence that she was acting with bad intent that's where would be in the first they weren't there look I just finish my answer senator. And so they came minutes that we can see thousands of emails. From the Clinton email domain including many many many. From the Verizon Clinton domain Blackberry domain. They said we think we got to get a search warrant to go get fees and the Department of Justice agreed we had to go to search war. So I agreed I authorize them to seek a search warrant and then I faced a choice and I've lived my entire career by the tradition. That if you can possibly avoid to avoid any action in the run up to election might have an impact whether it's a dog catcher election of president of the United States. But I sat there that morning and I could not see a door labeled no action here. I could see two doors and they were both actions one was labeled speak the other was labeled concealed. Because here's how I thought about not try to talk you into this but I want you know my thinking. Having repeatedly told this congress we are done and there's nothing there there's no case there is no case there. To restart. In a hugely significant way potentially find any emails that would reflect on her intent from the beginning. And not speak about it would require active concealment in my view. It's or stared at speak and conceals speak would be really bad. There's an election and eleven days Lordi that would be really bad concealing in my view would be catastrophic. Not just to the FBI but well beyond and honestly as between really bad and catastrophic. I said to my team we're got to walk into the world of really bad I've got to tell congress that we're re starting this not at some frivolous way in a hugely significant way. And the team also told me we cannot finish this work for the election. And then they worked night after night after night and they found thousands. Of new emails they found classified information on Anthony Weiner. Somehow. Her emails are being forwarded to Anthony Weiner including classified information by her assistant who met evident. And so they found thousands of new emails and and call me the Saturday night for the election and said thanks to the wizardry of our technology. We've only had to personally read 6000. We think we can finish tomorrow morning Sunday and so I met with them. And they said we found a lot of new stuff we did not find anything that changes our view. Of her intent. So were in the same place we were in July it hasn't changed our view and I asked them lots a questions and I said OK that's where you are but I also have to tell congress that were done. Look this terrible it makes me mildly and cautious to think that we might at some impact on the election. But honestly. It wouldn't change the decision everybody who disagrees with me has to come back to October 28 with me and stare at this and tell me what you would do. Which you speak for which you conceal. And I could be wrong. But we obviously made a decision between those two choices that even in hindsight it's been one of the world's most painful experiences. I would make that same decision I would not conceal that. On October 28 from the congress and I sent the letter to congress by the way people forget this either make a public announcement. I send a private letter to the chairs in the rankings the oversight committees did you know it's a distinction without a difference in the world of leaks. But it is he was very important and I tell them instead of concealing and regional people can disagree but that's the reason I made that choice. And it was a hard choice I still believe in retrospect the right choice as painful is this has been an upstart that the long answer.

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