Chief prosecutor Gerrie Nel also accused Pistorius of tailoring his testimony to fit the evidence at the scene. Pistorius denied the accusations.
Nel alleged that the Olympic runner changed his aim with his 9 mm pistol to ensure that he hit Steenkamp as she fell back against a magazine rack in a toilet cubicle. Pistorius said the claim was not true, one of many denials he has issued in four days of unrelenting cross-examination in a pivotal stage of the trial that is being broadcast on television and followed globally by people who once admired the double-amputee runner for his international achievements on the track.
Nel's methodical questioning put Pistorius under intense pressure, and the athlete sometimes became distressed, which in turn only prompted his accuser to ask him if he was using his emotional displays to mask his difficulty in answering the questions.
Pistorius testified he fired four times through the closed toilet door in his home last year after hearing a "wood" sound that he mistook for the door about to open. He said he thought an intruder was about to come out and attack him. Pistorius said that, in retrospect, the noise he probably heard was the magazine holder being moved by his girlfriend.
Nel said Pistorius killed Steenkamp intentionally after a fight and is lying about fearing an intruder. He said Pistorius heard Steenkamp, struck by the first shot, fall against the magazine rack and that he used that sound to adjust his aim.
"I'm saying you heard the magazine rack and you changed your aim," Nel said to the Paralympic champion. Steenkamp was hit by three of the shots, prosecutors say, while one missed.
"I wouldn't have heard anyone fall inside of the toilet while I was shooting," Pistorius replied to Nel, though directing his gaze to Judge Thokozile Masipa, who will decide on the verdict. The athlete faces 25 years to life in prison if convicted of premeditated murder.
He twice broke down crying, once when he was asked to repeat the order to leave that he says he shouted to the perceived intruder before firing through the door. Pistorius sobbed and wailed again when Nel insisted that he knew he was firing at his girlfriend, who was allegedly hiding from him after they fought in the pre-dawn hours of Feb. 14, 2013.
"I did not fire at Reeva," Pistorius said, crying as Masipa announced the day's second unscheduled break. After the judge left the courtroom, Pistorius stood sobbing with his body trembling and turned away from the gallery.
Nel also tried to pin Pistorius on whether he intended to shoot at the intruder, but Pistorius said he did not intend to do so and that he was terrified at the time.
"I didn't have time to think about what I wanted to do," Pistorius said.
That prompted Nel to question whether Pistorius was changing his legal strategy from "self-defense" to "involuntary action" and he later said Pistorius did not fire a warning shot. Pistorius said that when he shot he had no idea who was behind the door, in response to Nel's remark that he didn't know whether a child, an unarmed burglar or more than one person could have been in the toilet.
"You never gave them a chance, in your version," Nel said, building the prosecution case that Pistorius shot to kill.
Pistorius was also accused of tailoring his version to fit evidence at the scene, with Nel painstakingly listing alleged inconsistencies in the account and alleging that Pistorius had "concocted" his version of the shooting.
"You're tailoring your version as you're sitting there," Nel said. He accused Pistorius of being a stickler for detail on some matters, in contrast to his frequent statements on the witness stand that he could not remember aspects of his testimony.
Nel noted that Pistorius earlier said he warned Steenkamp to call police about an intruder in a whisper, contradicting later testimony that he spoke in a "low tone" and the prosecutor also said blood spatter evidence indicated that the athlete's statement about the location of 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 the carpet nearby show that it was on the floor before police arrived, and that its location amounts to evidence that the couple had been having an argument.
Torchia reported from Johannesburg.