(CNN) -- Oscar Pistorius faced another day of relentless cross-examination Friday as the prosecution challenged his account of the killing of his girlfriend, Reeva Steenkamp. 

Prosecutor Gerrie Nel has accused the athlete of hiding the truth about the death of Steenkamp, whom he shot last year through a closed toilet door in his home in Pretoria, South Africa. 

His questions again sought to undermine Pistorius' reliability and credibility and to portray the Olympic athlete as someone who was inventing his version of events and "tailoring" evidence to suit his story. 

As Nel turned once again to the early hours of Valentine's Day 2013, he repeatedly challenged Pistorius over his actions in the moments leading up to Steenkamp's death. 

The prosecution's argument is that Pistorius shot Steenkamp intentionally after a heated argument. Pistorius does not deny shooting her but insists that he mistook her for an intruder. 

Pistorius said he thought he heard the toilet door opening before he fired. 

"I didn't intend to shoot. My firearm was pointed at the door because that's where I believed that somebody was," he said. "When I heard a noise, I didn't have to think, and I fired -- I fired my weapon. It was an accident." 

Nel, known in South African legal circles for his bulldog-like approach to cross-examination, responded to Pistorius' testimony almost with scorn. 

"Your version is so improbable that nobody would ever think that it was reasonably, possibly true," he said. 

Nel then hammered Pistorius on whether he had known Steenkamp was in the toilet when he fired. 
Who was being questioned on Friday?
by who?
Why?
What was he hoping to accomplish with his questions?
Was Pistorious in a sport?
Who was the girlfriend?
How was she killed?
Where was he when he shot her?
did he intend to?
then why did he?
does he claim it was an accident?
what did he hear before he shot?
what kind?
was it the noise of the toilet door?
What kind of approach is the prosecutor known for?
Where?
what does he use that approach on?
How did he respond to the testimony?
why doesn't he think that anyone would believe the story?
what time frame did he continue to go back to with questioning?
Oscar Pistorius
Oscar Pistorius
Gerrie Nel
Prosecutor Gerrie Nel
about the killing of his girlfriend
killing of his girlfriend
he sought to undermine Pistorius' reliability
 sought to undermine Pistorius' reliability
yes
athlete
Reeva Steenkamp
 his girlfriend, Reeva Steenkamp.
shot
shoot.
bathroom
shot last year through a closed toilet door
no
I didn't intend to shoot
he mistook her for an intruder
mistook her for an intruder. 

yes
It was an accident.
a noise
I heard a noise
a noise
I heard a noise
yes
heard the toilet door opening
bulldog-like
bulldog-like approach
South African legal circles
South African legal circles
cross-examination
approach to cross-examination
with scorn
with scorn
it is improbable
version is so improbable
early hours of Valentine's Day 2013
early hours of Valentine's Day 2013