The double amputee sprinter, once revered across the world for his triumph over physical adversity, faces life in prison if convicted in the Pretoria High Court of the murder of Steenkamp, a 29-year-old law graduate and model.
Pistorius, known as 'Blade Runner' due to the prosthetics he wears on the track, says he shot Steenkamp in a tragic accident, firing at what he thought was an intruder hiding behind a toilet door in his luxury Pretoria home on February 14 last year.
Prosecutor Gerrie Nel, whose reputation as one of South Africa's toughest attorneys has earned him the nickname 'The Pitbull', cross-examined Pistorius while looking through photographs of the couple's bedroom taken after the shooting.
In a period of fierce questioning, Nel pointed out a series of objects in one picture which ran counter to Pistorius' account of events. In response, the sprinter accused the police of tampering with the scene but Nel ridiculed this suggestion.
"Let's sum up: A policeman moved the two fans, put the duvet on the floor, opened the curtains wider than they should be before the photographs were taken," Nel said.
"Your version is so improbable that nobody would ever think it's reasonably possibly true ... Your version is a lie."
Pistorius says after he shot Steenkamp, he ran out on to the balcony and screamed for help. However, in a picture shown to the court, a duvet and a large stand-up electrical fan clearly block his route to the balcony door.
With no direct witnesses, Nel's main task is to pick holes in Pistorius' testimony and cast doubt on his assertion he believed a burglar was in his house, a common fear in crime-obsessed South Africa.
Witnesses in nearby buildings have testified to hearing a woman's scream before the sound of shots, which the prosecution hopes will help prove that the couple had a heated argument before Pistorius intentionally killed Steenkamp.
"ANYBODY BUT YOURSELF"
During much of the 19-day trial, Nel has sought to portray Pistorius as an arrogant hot-head who is reckless with firearms and refuses to take responsibility for his actions.
"You will blame anybody but yourself," Nel said to the 27-year-old track star, cross-examining him about a separate incident in which Pistorius is accused of firing a pistol in a packed restaurant.
Pistorius said the gun was given to him by a friend under the restaurant table and went off by itself. Police Captain Christian Mangena gave evidence earlier in the trial, saying the weapon could only fire if the trigger was pulled.
The athlete said he could not explain how the gun went off and questioned his own defence advocate Barry Roux's decision not to cross-examine Mangena on his evidence.
"Now you blame counsel Mr Roux," Nel said, prompting Roux to shake his head at a colleague.
"You are lying," Nel said, holding Pistorius in a stare. "You just refuse to take responsibility for anything."
In a dramatic opening to his cross-examination on Wednesday, Nel shocked the Pretoria court when he confronted Pistorius with a graphic photograph of the dead Steenkamp showing the side and back of her skull, her hair matted with blood and brain tissue.
Pistorius broke down and sobbed as Nel pushed him repeatedly to take responsibility for killing Steenkamp.
