Transcript for George Zimmerman Trial; Inside the Fateful Fight
We begin with neighbor versus neighbor in the trial of george zimmerman. Dual eyeness withes testifying about what they saw and heard the night trayvon martin was killed. This was supposed to be the big opening week for the prosecution to make their case that zimmerman say murderer. So let's get right inside that courtroom to hear from abc's matt gutman on what happened today. Both the defense and the prosecution adjusting today over one of the pivotal questions of the case. Who attacked whom that night in february 2012? Reporter: John good had probably the best vantage point of the fight, his back porch, I could tell the person on the bottom had a lighter skin color. Reporter: That would have been george zimmerman, the neighborhood watchman. Good said martin was on top, straddling zimmerman. It looked like there were strikes being thrown, punches being thrown. Reporter: The defense's mark o'mara worked to reinforce the violence of that fight. The person you now know to be trayvon martin was on top, correct? Correct. He was the one reining blows down on george zimmerman? That's what it looked like. Reporter: Zimmerman, seen in this re-enactment video, said martin pounded his head against the pavement, something good testified he did not see. Felt like my head was going to explode. Was it going like this? I could not hear that, no. Could you hear this? No. Reporter: Seeing the men fight, good said he ran inside to call 911, then he heard the shot. You hear somebody yelling for help? I'm pretty sure the guy's dead out here. Holy . Moments later police arrived, cuffing zimmerman. What did he say? He said that he was yelling for help and that nobody would come help him. It was a bad, bad day for the prosecution, diane. But there was one point they kept hitting again and again. Yes, zimmerman was hurt. Yes, he was beaten up. But was it severe enough to warrant him shooting a 17-year-old boy police say he followed anyway in the first place. Thanks so much. Now bring in "nightline" anchor dan abrams. Bad day? What about this week for the prosecution? This was supposed to be the week where prosecutors proved to this jury that george zimmerman was the aggressor, he was the one who started it, he was the one on top when they were fighting. The problem is, that's not what the witnesses have said. The key witness today said zimmerman was pinned on the bottom. Does that mean self-defense automatically? This witness is a problem for prosecutors, not just because of what he said, but because of how he said it, so meticulous, very close, very careful in his testimony, but prosecutors will trnl hope that the jurors believe other witnesses with regard to who was on top. Then the witness on the phone with trayvon martin. Crucial witness. The last person to talk to trayvon martin. He said that trayvon's last words that she could hear were get off, get off. Which would suggest that it was zimmerman who was attacking trayvon martin. The problem was, there were inconsistencies in her story, again and again and which are going to be a problem for prosecutors. What do you think the prosecution has to get done next week? They have the burden of proof here. They have to prove this case. They have to step it up here. The lead investigator is going to have to explain why they didn't just charge him, but with murder with intent to kill. He'll be a crucial witness. All right, dan, thank you. Now we move to that
This transcript has been automatically generated and may not be 100% accurate.