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PRETORIA, South Africa (CNN) -- The legal battle over the fate of Oscar Pistorius focuses on police photographs of the crime scene Tuesday, a day after a gun dealer testified that the Olympic athlete knew it was both illegal and unsafe to shoot through a closed door.
That's what Pistorius did on Valentine's Day last year, killing his girlfriend Reeva Steenkamp.
He pleaded not guilty to murder, saying he thought she was an intruder.
But the testimony of gun dealer Sean Rens may rule out the possibility of acquittal, even if Judge Thokozile Masipa believes that Pistorius was genuinely mistaken about who was behind the door of the toilet room in his house.
Pistorius, 27, knew it was only legal in South Africa to fire at an intruder if he faced imminent threat, and that it was unsafe ever to fire his weapon when he could not see the target, Rens testified, citing a gun licensing questionnaire that the sprinter filled out.
The questionnaire included questions about what an intruder had to be doing before it was legal to open fire, and Pistorius answered correctly that a person was not, for example, allowed to shoot at someone only because he was breaking into a house.
It is legal to shoot a burglar if he is approaching you with a weapon, Rens said Pistorius knew.
The gun dealer was the latest of several witnesses to testify that Pistorius tended to grab his gun and head towards the potential threat when he felt there could be trouble.
Pistorius "went code red" once when Rens was with him, taking his gun and moving to clear his house room by room when he heard a suspicious noise, the gun expert said.
It turned out to be a tumble dryer.
The South African's apparent hair-trigger reflex could support his defense case that he heard a noise in the middle of the night after getting out of bed, did not realize that Steenkamp had also gotten out of bed, got his gun and shot her by accident.
Rens was in the process of selling two shotguns and an assault rifle among other guns to Pistorius was Steenkamp was killed, he testified, but the order was cancelled after she was shot. She was 29.
There's no question that Pistorius shot Steenkamp through a bathroom door in his house early on Valentine's Day last year, hitting her with three hollow-tipped bullets, one of which probably killed her almost instantly.
But for more than two weeks in court, his defense team has chipped away at every witness who casts doubt on his story that he thought Steenkamp was a burglar and mistakenly acted in what he believed was self-defense.
As the third week of the trial began, Rens took the stand, followed quickly by police photographer Bennie van Staden.
Prosecutor Gerrie Nel highlighted photos showing the position of spent bullet cartridges near the bathroom, blood spatter on a bedroom wall, and photos of Pistorius himself, covered in Steenkamp's blood on the night of the killing.
Steenkamp's mother June made a rare appearance in court Monday, but left as the photos of Pistorius were shown.
Van Staden also leafed through photos of Steenkamp's dead body on the witness stand, but those pictures were not shown on the monitors around the courtroom. Pistorius shaded his eyes as the photographer confirmed the pictures were the ones he had taken.
Pistorius threw up repeatedly last week as her injuries were described by the pathologist who did the autopsy, and again when the pictures were accidentally flashed on the screen as a computer operator scrolled quickly through crime scene photos.
Police took more than 900 pictures related to the killing, van Staden confirmed on Monday.
He will be back on the stand Tuesday for Barry Roux of the defense team to cross-examine.
Golden boy turned defendant
He's the 15th witness to testify for the prosecution in the murder trial of Pistorius, once South Africa's golden boy for the stellar track success that made him the first double amputee runner to compete in the Olympics.
The case against Pistorius is largely circumstantial, Nel said in his opening statement on March 3. Pistorius and Steenkamp were the only people in his house when he killed her.
Nel has been building a picture of what happened through the testimony of experts, neighbors who heard screaming and bangs that night, current and former friends of Pistorius' and a security guard who sped to the scene because of reports of gunshots.
Many prosecution witnesses' accounts are consistent with Pistorius' version of events: that he got up in the night, went out to his balcony to get a fan, came back inside and heard noises in the bathroom that he thought came from an intruder.
said he took the gun and fired while calling for Steenkamp to call police. When she didn't answer, he realized she could have been the person in the bathroom, he said.
Neighbors said they heard a woman screaming before the shots were fired. But the defense is proposing that what neighbors thought was Steenkamp screaming in fear for her life was in fact Pistorius when he realized what he had done.
And the defense says that the sounds neighbors heard were not the gunshots, but the cricket bat hitting the door as he tried to rescue her.
Pistorius and at least two neighbors made phone calls to security after the shooting, allowing the defense to use phone records to establish a timeline of events.
Judge Masipa will decide the verdict with the help of two lay people called assesors. South Africa does not have jury trials.
In South Africa, premeditated murder carries a mandatory life sentence with a minimum of 25 years. Pistorius also could get five years for each of two unrelated gun indictments and 15 years for a firearms charge he also faces.
If he isn't convicted of premeditated murder, the sprinter could face a lesser charge of culpable homicide, a crime based on negligence. The sentence for culpable homicide is at the judge's discretion. The trial is due to continue until April 4, take a break, and resume in mid-April.
CNN's Ed Payne contributed to this report
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