In a pummelling cross-examination, the prosecution called Oscar Pistorius's account of killing his girlfriend "impossible" and "a lie" on Thursday, highlighting apparent inconsistencies in key parts of his testimony.
"Your version is so improbable that nobody would ever think it's reasonably, possibly true it's so impossible," Prosecutor Gerrie Nel thundered during his second day interrogating the Olympian.
"Your version... is a lie," he insisted.
Nel also rubbished the athlete's claim that police moved important pieces of evidence after arriving at the scene early on Valentine's Day morning last year.
Under intense questioning, Pistorius said police moved fans, put the duvet on the floor and opened the curtains when they came to his villa, implying they had tampered with the crime scene.
"Is this one big conspiracy?" asked Nel with incredulity. "They would do all this to you?"
Pistorius, known as the "Blade Runner" for his j-shaped prosthetic legs, has been charged with murdering his 29-year-old model girlfriend after shooting her four times through a locked toilet door.
He faces a life sentence if convicted.
The double-amputee, once revered for his triumph over disability, has said he fired the shots accidentally and did not mean to kill anyone.
He also testified that he feared someone was coming through the toilet door and that his life was in danger.
This seemingly contradictory account was probed at length by Nel, who drew a concession from Pistorius that the shots should not have been fired at all.
"We know for a fact there were no intruders in your house that night, we know for a fact there was no ladder against the wall," Nel said.
"We know for a fact that you had no reason to shoot, objectively speaking."
Pistorius responded: "That's correct my lady."
Pistorius's cross-examination is a key point in his trial and a stern test of both his version of events and of his resolve.
During the five-week trial the world-famous athlete has appeared fragile, frequently crying in court and becoming physically sick when gruesome details of Steenkamp's injuries were discussed.
Regardless of who Pistorius believed was behind the door, he could face a stiff sentence if Judge Thokozile Masipa believes he purposely used lethal force without reasonable cause.
- 'All about I' -
Nel spent most of the first part of his cross-examination accusing Pistorius of being a selfish, controlling boyfriend and caring more about himself than the death of his girlfriend.
He tore into the 27-year-old's account of his relationship with Steenkamp, and read long cell phone messages in which the former model said she was upset and "scared" of Pistorius's behaviour.
"She's scared of the feelings that she has for me and the way that I brushed her off," the athlete said by way of explanation.
"It's all about 'I'. It's all about Mr Pistorius," Nel said.
When Nel asked what Steenkamp meant when she said Pistorius picked on her "incessantly," the runner got defensive.
"There are people who will testify I didn't pick on her in any way," he said.
Nel also accused Pistorius of making a public apology to Steenkamp's parents just to make himself feel better.
"Did you feel better after the apology?" Nel asked.
Steenkamp's mother June has said she wanted the Paralympian to see what he did to her daughter.
"He must see me there in the court, he must feel my eyes boring into him, I think it makes a lot of difference," she told Britain's Daily Mirror newspaper.
"I look at Oscar the whole time, to see how he is coping, how he is behaving. I'm obsessed with looking at him. It's just instinctive, I can't explain it."
After sitting in court with stoicism, June Steenkamp said she breaks down at the end of the day.
"I keep it all in and when I get back to the hotel it all comes out and I break down."
- Responsibility -
Nel also used questions about three separate gun charges to try and show that Pistorius, who has been described as a "gun enthusiast," was irresponsible.
Pistorius, who said in court he carried a firearm at all times, has denied firing a gun in public on two separate occasions, and the possession of unlicensed ammunition.
"I didn't pull the trigger. I didn't have time to think," he said about firing a gun in an upmarket Johannesburg restaurant in 2012.
"'I'm a gun enthusiast, I didn't have time to think,'" Nel mimicked sarcastically.
The athlete also said his counsel told him it was legal to keep his father's ammunition in his home safe.
"It's now the third occasion that you blame your legal team when you don't want to take responsibility," said Nel.
Pistorius earlier blamed his lawyers for discrepancies between his accounts given in written statements and his later verbal testimony.
Defence lawyer Barry Roux is expected to call up to 17 witnesses in the remainder of the case.
Originally scheduled to run for three weeks, the case has been extended until mid-May but could go on longer.