(CNN) -- Oscar Pistorius was the target of relentless and combative questioning Wednesday as the prosecutor in his murder trial tried to rattle the runner about the night he shot dead his model girlfriend.
Vowing to "get to the truth" of the events that took place on Valentine's Day last year, prosecutor Gerrie Nel began a blistering cross-examination of the Olympic sprinter in the case that has gripped South Africa.
"You shot and killed her. Say it -- 'I shot and killed Reeva Steenkamp,'" Nel told Pistorius.
The prosecutor immediately boxed the double-amputee athlete into a corner in what his defense team described as an "ambush," by asking him about a video showing Pistorius shooting a watermelon at a gun range.
No one disputes that Pistorius killed Steenkamp. But the prosecution is trying to prove beyond a reasonable doubt that he did so knowingly and intentionally.
The 27-year old has admitted to the killing but said he had mistaken Steenkamp for an intruder in the bathroom when he fired through the door and killed her.
Before Nel went after Pistorius, Barry Roux -- the runner's defense lawyer -- had tossed his client a question to drive that argument home. He asked Pistorius if he had intentionally killed Steenkamp.
"I did not intend to kill Reeva or anybody else for that matter," Pistorius replied.
Later in the proceedings, as Nel probed him further, Pistorius insisted he thought he would be attacked when he heard noise coming from his bathroom that night.
"I had a fear, I didn't have time to think, I discharged my firearm ... I didn't intend to shoot at anyone, I shot out of fear."
Court was adjourned for the day following that testimony.
Minutes after starting his cross-examination, Nel took the defense team by surprise when he asked the athlete about a shooting range video in which Pistorius is seen firing at a watermelon, and then calling the impact "a zombie-stopper."
This prompted the defense to complain that the prosecution was staging an "ambush" by introducing evidence. The court was briefly adjourned as Judge Thokozile Masipa considered both positions. The defense later said it would not object to the video being shown in court.
"It makes me very upset to hear myself saying something like that," Pistorius said as he admitted to making the comment. But he insisted he was referring "to a zombie, not a human being."
Nel showed the court a graphic photo of Steenkamp's wounded head. Speaking of the watermelon in the video, the prosecutor said: "It exploded. You know the same happened to Reeva?"
"I was there, I don't have to look at a picture," Pistorius snapped, sobbing.
Roux objected to Nel's comparison, and the judge agreed, adjourning proceedings to give Pistorius time to recover. His sister, Aimee, stood next to him to console him while he held his head in his hands and continued crying.
'My life is on the line'
Upon Pistorius' return, Nel pressed the runner on whether his recollection of that night was a reconstruction of what he heard and read rather than what he remembered. Pistorius said it is what he remembers.
"My version has never changed," he said.
Nel went over with a fine-tooth comb the version of events that Pistorius provided in an earlier bail application affidavit, and the version he has given on the witness stand over the past three days.
He repeatedly asked the athlete about the location of two ventilation fans in his bedroom on the night of the killing, seeking to undermine the defense's contention that the crime scene was contaminated by clumsy police work and other factors.
The detail was important, Nel told Pistorius, because "it will show that you are lying."
Pistorius said his memory was not good but that he was not trying to lie. "I can't change the truth," he said.
The athlete conceded there is a discrepancy between his bail application and his court testimony, saying that a claim on the bail application that he went onto a balcony to retrieve a fan before the shooting was wrong. He added that he was not present when the bail application affidavit was written.
Nel, who accused the runner of not answering questions directly at times, also asked Pistorius whether he was thinking of the implications of his answers. Pistorius said it would be reckless not to think of what he said, adding: "My life is on the line."
"Reeva doesn't have a life anymore," the prosecutor said.
The night of the killing
At the beginning of the day, Pistorius finished telling the court what he remembered from the night Steenkamp died, starting with the moment he opened his bathroom door after shooting through it and saw her bloodied body.
His voice quivering, the track star picked up where he left off the previous day, when he became so upset by his own testimony about that night that he broke down into uncontrollable sobs on the stand.
Steenkamp was sitting slumped over the toilet bowl, Pistorius said while being questioned by his defense lawyer. He said he pulled her weight onto him and cried. That's when her blood streamed down on him, he said.
Pistorius had grabbed Steenkamp's cell phone, but didn't know the pass code, so he went to get his own to call for help, he said.
He called a neighbor, Johan Stander, who arrived soon after. The athlete said he shouted and screamed at Stander to help him get Reeva to the hospital.
Then another neighbor, Dr. Johann Stipp, a physician, arrived at the house. The doctor seemed overwhelmed by the situation, Pistorius said.
The athlete also described the moments after paramedics had arrived at the scene.
"Reeva had already died while I was holding her, before the ambulance arrived," he said. "I knew there was nothing more they could do for her."
He also spoke of when police officers arrived at his door -- one of them a key witness in his trial, former Col. G.S. van Rensburg, Pistorius said.
"Every time I looked up there were more people in the house. There were more policemen. There were people going up and down the stairs. I was standing in the kitchen. I asked a policeman if I may wash my hands because the smell of blood was making me throw up."
After pictures were taken, police told him he was under arrest and would face charges.
On his first day on the stand, Monday, Pistorius made a tearful apology to Steenkamp's family.
The prosecution alleges Pistorius killed his girlfriend after they argued.
Roux had also delved deeper into his connection with Steenkamp, trying to show they had a loving relationship. The couple met a little more than three months before Steenkamp died at the age of 29.
In affectionate messages read out in court, the pair used pet names like "baba" and "angel," said they missed each other and exchanged many "x"s, or kisses.
Only those in the courtroom can see Pistorius because he has chosen not to testify on camera. His testimony can be heard on an audio feed.
Steenkamp's mother, June, has sat in court throughout Pistorius' three days on the stand.
The defense team will call 14 to 17 witnesses, Roux said as he opened his case. The trial is scheduled to continue until the middle of May.
Judge Thokozile Masipa will decide the verdict in collaboration with two experts called assessors. South Africa does not have jury trials.
CNN's Richard Allen Greene, Brent Swails, Emily Smith and Ben Brumfield contributed to this report