(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.

1. Who was being questioned on Friday?
2. by who?
3. Why?
4. What was he hoping to accomplish with his questions?
5. Was Pistorious in a sport?
6. Who was the girlfriend?
7. How was she killed?
8. Where was he when he shot her?
9. did he intend to?
10. then why did he?
11. does he claim it was an accident?
12. what did he hear before he shot?
13. what kind?
14. was it the noise of the toilet door?
15. What kind of approach is the prosecutor known for?
16. Where?
17. what does he use that approach on?
18. How did he respond to the testimony?
19. why doesn't he think that anyone would believe the story?
20. what time frame did he continue to go back to with questioning?
1. Oscar Pistorius
2. Gerrie Nel
3. about the killing of his girlfriend
4. he sought to undermine Pistorius' reliability
5. yes
6. Reeva Steenkamp
7. shot
8. bathroom
9. no
10. he mistook her for an intruder
11. yes
12. a noise
13. a noise
14. yes
15. bulldog-like
16. South African legal circles
17. cross-examination
18. with scorn
19. it is improbable
20. early hours of Valentine's Day 2013