Transcript for Chicago police officer who killed Laquan McDonald testifies in own defense
Reporter: It's these tense moments from 2014 now under scrutiny, chilling dash cam video showing a Chicago police officer shooting 17-year-old laquan McDonald 16 times, killing him. Today, that officer, Jason van Dyke, taking the stand in his own defense. His face was just expressionless, turned his torso toward me. Reporter: Charged with first-degree murder, Van Dyke testifying he feared for his life and fired only after McDonald ignored repeated commands to drop a three-inch knife during the incident. I was yelling at him, drop the knife. I yelled it I don't know how many times but that's all I yelled. Reporter: In the four years since the shooting, this case has become a flash point in the ongoing tensions between police and the black community in America. Prompting nationwide protests. We just want justice. We're out here marching and protesting to make sure it doesn't happen again. Reporter: It was October 20, 2014, when Van Dyke responded to a call about someone with a knife trying to break into vehicles. McDonald was holding a small knife in his right hand. Police dash cam video shows van Dyke getting out of his squad car and opening fire within six seconds. Autopsy reports would show McDonald had the drug pcp in his system the night he was killed. From the beginning, Van Dyke has maintained he acted in self-defense. Chicago mayor Rahm Emanuel and city attorneys fought to keep the video from becoming public, but just over a year later, a judge ordered the video be released. Van Dyke was charged with first-degree murder the same day. It is graphic. It is violent. It is chilling. Reporter: Van Dyke's defense argues he feared for his life when he opened fire. This is not a murder case. Despite what you heard in the courtroom. I would definitely call it excessive force based on the dash cam video. Mr. McDonald is walking away from the officer and posed no immediate threat. Best practices for using deadly force is as a last resort. Reporter: The video of McDonald's death sparked months-long protests in Chicago and across the country. 16 shots. The sooner you can release information, the better for the community, and because there was such a delay, for whatever reason, the delay really just builds more distrust of law enforcement. Reporter: As community tensions ran high, Chicago's top brass was on edge. Jason Van Dyke will be judged in a court of law. That's exactly how it should be. Reporter: At the time, ABC news confirmed 18 complaints filed by citizens against van Dyke since he joined the force in 2001, including allegations of using excessive force and racial slurs. But Van Dyke was not disciplined. If we don't get it, shut it down. Reporter: Under public pressure, Emanuel fired then Chicago police superintendent Gary Mccarthy. Now's a time for fresh eyes and new leadership. Will everybody please be seated. Reporter: Finally, last month, the long anticipated trial. Defense call your next witness, please. Reporter: Today, an emotional Jason Van Dyke explaining what happened in his own words. He waved the knife from his lower right side upwards, across his body, towards my left shoulder. And when he did that, what did you do, officer? I shot him. Reporter: What Jason van Dyke's testimony did is tell jurors there's more to the story than the video and that's exactly what Jason Van Dyke needed to get across. Reporter: Defense attorney saying this was self-defense, using a measuring tape to mark the 13 feet between the two men. An expert in police use of force showing how quickly McDonald could have come at Van Dyke with the knife. And I'll show you how much time you would have to react to me. Reporter: The defense also submitting an animation aimed to depict the officer's perspective and counter the police dash cam video. Ultimately, Jason Van Dyke's defense is that he was justified when he pulled the trigger because he believed that laquan McDonald was moving the knife from his waist across his chest and up over his left shoulder. That's what Jason Van Dyke testified, and Jason Van Dyke believed that was enough of a threat to return force with force. Reporter: But prosecutors wondered why if Van Dyke felt threatened he's seen walking towards McDonald. You had six seconds before you pulled the trigger the first time. Right? And in that six seconds, he got a lot closer to me than I could have gotten away. And you got a lot closer to him too, didn't you? I know that now, yeah. Not intentionally. I thought I was back pedaling that night. You thought you were back pedaling as you're firing shot after shot after shot? What I know now and what I thought at that time are two different things. Reporter: Prosecutors relying heavily on the video, playing it dozens of times throughout the trial. Can you see that? Reporter: Arguing the officer's use of deadly force was unjustified. Did you ever make a decision to stop shooting that night? Yes, I did. And when was that? Once I recognized that he hit the ground. Well, he hit the ground and you continued to shoot, correct? Reporter: And pressing on why he kept shooting after McDonald collapsed, Van Dyke at first saying he thought McDonald was trying to get up. Was there any point in that video where laquan McDonald was trying to get up? In that video, it may not show it but that wasn't from my perspective, ma'am. I was coming at a completely different angle. Reporter: He also says he continued to fire, aiming at the knife. So why would you continue to shoot at his knife? That's not what you're trained to do. My focus was just on that knife and I just wanted him to get rid of that knife. That's all I could think. I think the most striking thing about this incident is the number of shots. We're trained, as cops, to fire several volleys and assess. It ended up being 16 shots. That's beyond the pale of what's needed to stop a suspect. Reporter: Jurors in this case are probably going to focus not on the number of shots fired, not on the number of seconds that ticked by between this defendant's "Arrival" and when he pulled the trigger but rather jurors are probably going to look at what the victim was doing in the split seconds before officer Van Dyke pulled the trigger, and then, if jurors believe that the officer was justified in perceiving that threat as a serious threat, then this officer walks. Reporter: It's that question that makes these types of cases notoriously difficult to prosecute. Each instance unique, but the common thread? An officer who felt their life was in danger. A grand jury decided not to indict Timothy allowman, who shot and killed tamir rice. Betty Shelby was found not guilty and not guilty in the death of philando Castille. If convicted, Van Dyke could face life in prison. The case could go to the jury as early as next week. I'm Alex Perez in Chicago.
This transcript has been automatically generated and may not be 100% accurate.