His voice quavering at times, Pistorius struggled to explain alleged inconsistencies in his testimony and broke down sobbing on two occasions, forcing Judge Thokozile Masipa to temporarily halt proceedings.
Prosecutor Gerrie Nel was sometimes quick to acknowledge Pistorius' distress — possibly to allow him time to recover and avoid any defense argument that he is not getting a fair trial — but also said the athlete was frantically trying to shore up a fabricated story.
"You're getting frustrated because your version is improbable," Nel said, standing at a lectern and gesturing with his spectacles in his right hand. "You're not using your emotional state as an escape, are you?"
Pistorius said he wasn't in a "rational frame of mind" at the time of the shooting in his home in the early hours of Feb. 14, 2013, suggesting he was therefore unable to remember some things about that night or explain some of his actions, such as rushing around with a cocked gun after he killed Steenkamp.
The cross-examination, which resumes for a fifth day Tuesday, is at a pivotal stage in a trial watched on television around the world by viewers who had admired the double-amputee runner for his track achievements. Once a role model with lucrative sponsorship deals, Pistorius is now a suspect in a witness box, challenged by an accuser in a black robe.
Pistorius faces 25 years to life in prison if convicted of premeditated murder, and Nel's challenge is to prove the state's case beyond a reasonable doubt. Meticulously, he has sought to pick apart the runner's account, exposing what he describes as a pattern of improbabilities that, taken as a whole, prove Pistorius is lying when he says he mistakenly shot Steenkamp through the closed door of a toilet cubicle because he feared an intruder was inside.
Nel, who alleges that Pistorius killed his lover after an argument, noted that the athlete earlier said he whispered to Steenkamp to call police about an intruder, contradicting later testimony that he warned her in a "low tone." The prosecutor also said blood spatter evidence indicated that the athlete's statement about a duvet in the bedroom was false.
Pistorius has said the duvet was on the bed, and that police photographs of the bed cover on the floor suggest that police moved it there after the shooting. Nel said a pattern of blood drops on the duvet and on a nearby carpet show that it was on the floor before police arrived, and that its location amounts to evidence that the couple were arguing.
Nel also challenged Pistorius about his version of events after the shooting, asking why he didn't check outside a bathroom window to see if there might be a ladder used by a perceived intruder to gain entry, and why he didn't check to see if Steenkamp might have fled through the bedroom to another part of the house before concluding — in a "huge leap" of logic — that he had mistakenly shot her.
"It didn't even cross my mind to check the bedroom door," Pistorius replied, saying he was entirely focused on the possibility that Steenkamp was in the toilet cubicle.
Pistorius testified he fired four shots through the toilet door after hearing a "wood" sound that he mistook for the door being opened by a possible assailant. Pistorius said that, in retrospect, he probably heard a magazine holder being moved by his girlfriend.
He denied Nel's allegation that he heard Steenkamp fall against the magazine rack after being hit by the first shot, and used the noise to adjust his aim to make sure he hit her again. Steenkamp was hit by three shots.
The prosecutor tried to pin down Pistorius on whether he intended to shoot at the perceived intruder.
"I didn't have time to think about what I wanted to do," Pistorius replied, prompting Nel to question whether Pistorius was changing his legal strategy from "self-defense" to "involuntary action."
Pistorius said he shouted for any intruder or intruders to get out of his house and that he had no idea who was behind the toilet door when he fired. Nel remarked that, according to Pistorius' account, he didn't know whether a child, an unarmed burglar or more than one person could have been in the toilet stall.
"You never gave them a chance, in your version," Nel said, noting that he never fired a warning shot. The truth, the prosecutor argued, is that Pistorius shot to kill — and that his target was Steenkamp.
Torchia reported from Johannesburg.
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