The fast times of oscar pistorius continues. Once again, amy robach. Reporter: As the sun set on PRETORIA ON FEBRUARY 13th, NO One knew that the next day, the city, and the world, would awake to the...
See More 
The fast times of oscar pistorius continues. Once again, amy robach. Reporter: As the sun set on PRETORIA ON FEBRUARY 13th, NO One knew that the next day, the city, and the world, would awake to the news of a valentine's day blood bath. Breaking news at this hour. Paralympian oscar pistorius has been arrested for -- his girlfriend was shot dead at his home. An olympic hero charged with murder. Reeva steenkamp had been shot to death in oscar pistorius' house, and south africa's golden boy was under arrest. I didn't believe it. I phoned a few of our friends, mutual friends. I started crying. I started screaming. I can't even tell you how many times I told reeva that I can't live without her. Reporter: The story goes like this. Four bullets fired through a bathroom door. On a day designed for romance, a girlfriend dead. Pistorius tells lead police investigator hilton botha -- coincidentally, the same officer who'd arrested pistorius in that 2009 incident -- that it was all an accident. Pistorius says he mistook steenkamp for an intruder and accidentally shot her. But the cops don't buy it. Every single policeman i spoke to has told me that that man is going to jail for a very long time. Reporter: Prosecutors charge pistorius with premeditated murder. And last week, the blade runner appeared in court for what could reasonably be called the bail hearing of the century. It's not since i reported on the release of nelson mandela and the fall of apartheid that I've seen so many foreign journalists in south africa. Journalists arrived early -- as you can look behind me, there's a lot of local and international media who have a high interest in this case. Reporter: This is the courtroom. His is the very place where oscar pistorius went from international superstar to murder suspect. Neither side is saying he didn't kill reeva steenkamp. No one in court is going to say he didn't pull the trigger. Reporter: It's just a matter of why. How and why. We hope that justice will prevail. Reporter: Over the course of four days, prosecutors lay out their case. Much anticipation outside the pretoria magistrate's court -- it has a tight case to support the charges. Reporter: You're confident? We are confident, yeah. Reporter: Prosecutors here are taking the position that this was a premeditated murder, that he was targeting his girlfriend, and that there was no ambiguity about it. Reporter: It all began february 13, just before 6:00 Steenkamp drives to oscar pistorius' luxury home. This is the last known image of her, first aired on south african tv. Ten minutes later, pistorius follows in his white bmw. Later that night, steenkamp texts her best friend's family, "i'm sleeping at oscar's tonight." When she texted you all that night, and said she was spending the night -- mm-hmm. You didn't think anything of it. Not at all. Reporter: What happened next? The prosecution and defense have two radically different theories. So we decided to examine them. We built a life-sized model of pistorius' home, based on the floor plans released in court. What is the version of events that night according to the prosecution? Prosecutors believe there was a fight, that witnesses could hear the fight from outside these windows that were open. Possibly for up to an hour. They believe reeva then flees to the bathroom. He puts on his prosthetic legs. At some point, comes around to this side of the bed, grabs his gun from underneath the bed, -- and then goes after reeva? Chases her into the bathroom. Reporter: This is quite some distance from the bedroom to the bathroom. That's right. And that's why prosecutors believe they have premeditation. Reporter: Pistorius fires four times. Three shots hit steenkamp, in the hip and elbow, and in the head. You open this door to see where reeva was found. It's pretty chilling. She had nowhere to go. And the other question becomes, if an intruder is trapped in this small space, why is oscar pistorius so intent? Why does he feel the need -- why does he feel atened? To shoot inside without figuring out who is in here first? Then there's the controversial cricket bat which has damned him in the press. A newspaper says democrats are working on the theory that reeva steen camp may have been assaulted with the bat before being shot four times. Reporter: The question remains, exactly what was this used for? The autopsy has not been released, so we don't know exactly how this play in the case. Pistorius says he used it to break down the bathroom door. Then he says he carried steenkamp from the bathroom, down the stairs, desperately trying to resuscitate her with the so called "kiss of life." Why was reeva moved? What was her state? Was she alive at the time? Did she say anything? That will come out in due course. 4:15 a.M., Police arrive at the scene. Steenkamp is already dead. While the tough guy weeps in the garage, police search for clues, finding two phones by the bathmat, one splattered with blood. The prosecution calls the location of those phones, the key to the case. Prosecutors say, what were two cellphones doing there? It would imply an argument. She just got up in the middle of the night and brought two cellphones with her to leave outside the toilet. They say it doesn't make sense at all. There's allegations of a message to reeva that may have made him lose his temper. Reporter: Was this the motive for murder? Did a combination of jealously, rage and easy access to weapons result in reeva's bloody death? The police have confiscated the cell phones involved to see if there is any evidence on those cell phones to confirm those allegations. Reporter: By the left side of the bed, police find steenkamp's overnight bag and slippers. Also the holster of the gun used to kill her. Prosecutors argue that if pistorius thought there was an intruder, he should have noticed his girlfriend was not in bed. They're saying, he goes down, gets the gun from underneath the bed and doesn't notice that she's not here in her usual spot? Reporter: The defense says they switched sides that evening. That's right. He's saying. I was sleeping on this side that night. Even if that's true, the question remains, how did she -- how did he not notice that she wasn't in the bed? Reporter: Worldwide press had run rampant with every detail of the crime scene, like syringes and a substance found at the scene. This could have been some sort of a roid rage. There's word now that the 26-year-old olympics athlete will reportedly be tested for steroids. Reporter: The prosecution knows the world is watching. They are confident that they will prevail. A plea bargain is highly unlikely. Highly unlikely. Reporter: You think this is going to trial? Yes. Reporter: A slam-dunk case? Maybe. Maybe not. And ironically, the fate of blade runner oscar pistorius may hinge on his most famous feature -- his missing limbs. They're convinced he did put on his legs. Reporter: Which suggests premeditation? If he's putting his legs on to go do that, absolutely. Reporter: Could his prosthetic legs become the evidence that decides the case?
This transcript has been automatically generated and may not be 100% accurate.
