Oscar Pistorius Trial: Forensics Take Center Stage

Prosecution and defense offer different theories in analyzing bullet holes in the bathroom door.
3:00 | 03/13/14

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Transcript for Oscar Pistorius Trial: Forensics Take Center Stage
But now we move our attention to South Africa where a new day is about to begin in court for the so-called blade runner, Oscar pistori Pistorius. On trial for allegedly murdering his girlfriend. Dramatic moments as the night of the crime is re-enact the by law enforcement. Matt Gutman is in Pretoria with the latest. With the repeated action of a cricket band, the prosecution tried to pound holes in Oscar Pistorius' version of events that he killed his model girlfriend reeva steenkamp. He said when he realized it was a lover, not intruder, he battered it shutdown with this bat. A police forensic expert took the stand for the prosecution, asking to demonstrate how the door was smashed in. That door is crucial evidence says Dan Abrams. It tells the story of w406s where, where the bullets came from, where Oscar Pistorius was when he swung this cricket bat. And prosecutors believe it helps them demonstrate he was lying. He says he went back to his bedroom after the shooting, screaming for help, puts his prosthetic legs on, goes back and then beats the door down. Prosecutors say that's not what the evidence and the door show. Reporter: He was now being painted as a liar, a murderer. For nearly a decade he had been an inspiration to south africans and to millions across the world. Pistorius was born without fibulas. So to aid his mobility his parents had his legs amputated below the knee when he was a child. Pistorius never wallowed even when his mother tragically died when he was just 15. With those prosthetic legs he would go on to dominate the special olympics in@then in 2004. Oscar Pistorius is flying away from all of them. Then in 2012 after years of lob lobbying, he was allowed to compete with able Bies athletes in the London games. For blind beauty and reality star, it seemed like a storybook love affair. She was happy. As far as I knew, she was happy. The royal couple played well in the tabloids. Their passion seemingly evident. Very quickly, reeva steenkamp started spending more and more time at his home. And then Valentine's day 2013, that shooting, steenkamp was dead and Pistorius was charged with murder. And for the past eight days in court, the blade runner has sat there on the front bench. The prosecution describing him as an egomaniac, obsessed with fast cars, guns, and perhaps his girlfriend. But from the start, he insisted he was innocent. Not guilty, my lady. He's been solitary but never still. So often anguished. And on Friday and Monday, a visceral reaction to the pathologist's testimony. I tried to open the airway and found no signs of life. First came the heaving, then the wretching. This bucket slid over to him. Then that handkerchief covering the clammy face. Early we heard the ear witness accounts of the screams heard from the Pistorius home late last night. Petrifying screams. I heard that female screaming fearfully. The frequent swings at the bullet-ridden door. But under cross-examination. But then, it wasn't the door that seemed to be dismantled but the police credibility. The defense is trying to show they messed up this investigation. The evidence wasn't cared for. There may have even been corrupt police officers involved in this case. Reer Reporter: The prosecution and defense have two very different versions of what happened. So ABC news built a lifestyle model of Pistorius' home based on the floor plans. That's Amy robach and Dan Abrams. Prosecutors believe there was a fight that witnesses could hear the fight from outside these windows that were open. They believe reeva then flees to the bathroom. He comes around to this side of the bed, grabs his gun -- And then goes after reeva. Pistorius fires four times. Three hit steenkamp in the hip, elbow and the head. He said he thought it was an intruder, but even his version of events in which he insists he's innocent of murder could still put him in hot water. Even if you mistakeningly kill someone, 1% guilty you could be guilty of culpable homicide. He's walked out this door eight times before through this gauntlet of media. But today may have been his toughest day of court. But outside every day, he's greeted with rock star squeals. Is this the first time you've ever been able to see him? Yeah. Reporter: Yesterday we caught up with a group of girls there for their first glimpse. You watch it all day long, right? 24 hours of Oscar Pistorius. Can you get enough? No. Reporter: We got an inside look at that channel, pumping out 15 hours of all things Pistorius every day. John Webb is one of its anchors. How obsessed are south africans with this story? I think they're becoming more obsessed. This is all new territory for many south africans. We haven't had this kind of access to court proceedings before. Especially around a criminal trial of this significance. I think the more it goes along, I think the more interested people are coming. And it will peak when the blade runner himself finally takes the stand to explain how he took his love's life. I'm Matt Gutman for "Nightline" in Pretoria, South Africa. We will, of course, stay on the story.

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

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