Reporter: And now, benghazi. We have talked to experts and historians who say this Frack shus issue could be decisive, making the difference in any Hillary Clinton campaign, raising questions about... See More
Reporter: And now, benghazi. We have talked to experts and historians who say this Frack shus issue could be decisive, making the difference in any Hillary Clinton campaign, raising questions about leadership in a crises. After 13 hearings on the topic, 25,000 documents have been released, the fierce debate over responsibility rages on. So we begin now on that night, her friend, ambassador Chris Stevens, and three other Americans in Libya lose their lives. Put yourself in the place of the American ambassador on the night of September 11, 2012. Chris Stevens is in a small outpost in the north eastern city of benghazi. He's heading off to bed when suddenly, look at this video, dozens of men coming towards him with automatic rifles and rocket powered again aides. He makes his way into a fortified room, locking the steel doors. A seasoned ambassador, known for his big optimistic smile, fighting for his life. He spoke Arab bik, was passionate about giving Libyans a better future. I look forward to watching Libya develop equally strong institutions of government. Reporter: But as he and his aid hide in the fortified room, the attackers set fire to the building. Stevens suffocates in the smoke. Two former Navy S.E.A.L.S also die that night, taken down by mortar bombs. In the interview, I show secretary Clinton a picture of Chris Stevens, the man she chose to be ambassador to Libya. Here's Chris with that amazing smile. The hardest part is to think about SHAWN smith and Chris Stevens being trapped, and when I think about that, it just breaks my heart because the diplomatic security personnel were performing heroically. Reporter: But this is the central asked by her critics, did she do everything she could or should have done to make that outpost safe on the anniversary of 911? His last words in his diary, Chris Stevens, never-ending security threats. Right. But he was there on 9/11. Right. And of his own choosing. But you wanted a post there? It was important to have diplomatic assets. Were there security threats? Yes. There's a long list of countries where there are security threats to American interests. But these were the highest among the highest? Well, it would be in the top 25. It was not -- Not in the top five, top ten, benghazi? Maybe in the top upper ten, but there were places where we had much more concern. Reporter: But should she have known the situation in benghazi was deteriorating fast? A cable in August a month before the attack warnings about vulnerability. There had been two bomb attacks on the mission in the last six months. The red cross had pulled out of benghazi, the british had left, too. You know the criticism is, it was a glaring, flashing red crises there. The british were pulling out. There had already been attacks. There were cables being sent. Did you miss it? Did you miss the moment to prevent this from happening? No. But I think as the independent board that investigated every aspect of this including all the cables, concluded there was a lack of appreciation and response to the level of threat. Now, there were a lot of -- By you, too? Well, no. There was never brought to me. Reporter: She says she did not see that August cable, but she relied on her staff, the security professionals, to do the job. Is there anything you personally should have been doing to make it safer in benghazi? What I did was give very direct instructions that the people who have the expertise and experience in security -- But personally, you personally. That's personal, Diane. I'm not equipped to sit and look at blueprints to determine where the blast walls need to be. That's why we hire people that have that expertise. The mission was far short of standards, weak perimeter, incomplete fence, video surveillance, needed repair. They said it's a systemic failure. Well, it was with respect to that compound, and it was not a permanent facility. The annex was much stronger and had greater security. Two men died there. The chairman of the joints chiefs of staff said he was surprised the secretary of state did not know about that August cable? Are you stunned that she didn't? I would call myself surprised. Reporter: She insists that delegating security to the staff didn't mean she wasn't concerned. But that doesn't mean I wasn't saying all the time we've got to make sure we secure this spot and that spot. I wonder if people are looking for a sentence that begins from you, I should have, I should have. We saw your face on that tarmac. Something that said I should have done this differently. I would give anything on the Earth to personally if I could have done this differently. Well, I certainly would give anything on Earth if this had not happened, and I certainly would wish that we had made some of the changes that came to our attention to make as a result of the investigation. But I also am clear this my own mind that we had a system and that system of course ended with me. But I take responsibility, but I was not making security decisions. I think it would be a mistake for a secretary of state to sit and say, okay, let's go through all 270 posts and let me decide what should be done. That, to me, is inappropriate where the experience and the expertise lies elsewhere. But the top ten? Top ten, but it's a constantly changing scenario. Are you saying it's just the price of doing business to have people in dangerous outposts even with less than the adequate security that the review boards have said they needed? I'm saying that we have to be very thoughtful as United States of America where we send people, why we send them, what we expect from them, and how we do the best to protect them. We cannot eliminate every threat, every danger. Reporter: But there's still another part of the story that has created a firestorm of doubt and criticism. Those first statements by the administration, talking about attacks as part of a wave of protestors out of control. Critics believe they were a diversion designed to protect the campaigning president from the charge he failed to stop a terrorist attack on his watch. But you're also eight weeks before an election. Well -- This is politics. Well, I'm shocked that policy and politics are at work in Washington, but that's not what was going on. What was going on was trying in the midst of what, frankly, was the fog of war to figure out what had happened. You famously said it was a protest or a group of guys deciding to kill Americans -- What difference, at this point, does it makes. That's right. Does it make a difference? In the moment it did not. In the moment what we had to be focused on was saving American lives. Does it make a difference now? Do you want to change that? No, I don't. Because the point of what I said at the time was, you know, if you are going to stay fixated on things like talking points or fixated on whether or not everybody was affected or not by the video, you're missing the larger picture. Reporter: As we said, after two years and all those hearings, all those documents, republicans in the house have now announced yet another investigation of benghazi and Clinton to begin later this year. Are you going to testify? Well, that's going to be up to the people running the hearings. If they ask you? We'll see what they decide to do, how they conduct themselves. But what I do not appreciate is politicizing this at the expense of four dead Americans. That's not what we used to do in this country. 258 Americans were killed in Beirut in two separate attacks. People mourned, people were shocked. Decisions were made, bring them out, strengthen the embassy. Is that another reason not to run, too much? No. Actually, it's more of a reason to run. I do not believe our great country should be playing minor league ball. We ought to be in the majors. I view this as really apart from, even a diversion from, the hard work that the congress should be doing about the problems facing our country and the world. Reporter: We know so many of you are weighing in online right now. Join our group of experts and historians as they weigh in, too. Tweet us, go on Facebook, we want to hear what you think. Tomorrow on "World news" you will hear from our experts and some of what all of you had to say tonight.
This transcript has been automatically generated and may not be 100% accurate.