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PRETORIA, South Africa (CNN) -- For a second day, Oscar Pistorius' lawyer Thursday grilled a police expert who demonstrated a day before how the Olympic runner bashed a bathroom door with a cricket bat after killing his girlfriend Reeva Steenkamp.
With fastidiously detailed questions, defense attorney Barry Roux picked at forensic expert J.G. Vermeulen's testimony on the validity of evidence presented by the door in the Olympic sprinter's murder.
Pieces of it are missing, and Roux asked Vermeulen to account for them. He painstakingly sifted through memory and past details, while shuffling point for point through documents.
Casting doubt on the credibility of the door as evidence -- and Vermeulen's assessment of it -- is in Pistorius' interest, since they support the prosecution's case against him.
Pistorius wore a blank stare during Roux' exacting cross-examination, looking tired, uninvolved and not making as many notes, as he did in previous sessions.
The door difference
Prosecution and defense agree that the South African track star killed his girlfriend on Valentine's Day last year when he shot four hollow-tipped bullets through the locked bathroom door.
But while the state argues it was deliberate murder, Pistorius says he thought she was an intruder in his house in the middle of the night and the killing was a tragic accident.
Pistorius, 27, claims that after he realized his mistake, he put his legs on, took his bat and whacked the door to get to Steenkamp, 29.
That's where his account differs from the prosecution's arguments. They say he was on his stumps, when he swung the bat.
On Wednesday, Vermeulen provided some of the trial's most dramatic testimony by swinging Pistorius' own bat at the very door from Pistorius' home, assembled in court for the judge to see.
The police colonel wielded it from different distances and angles, even on his knees to simulate the height of Pistorius, a double amputee, without his prosthetic legs on.
The officer concluded that Pistorius was on the stumps of his legs when he battered the door. It was a surprising change in stance. Previously, prosecutors had argued the opposite.
If Vermeulen's testimony stands up, it could look as though Pistorius was lying.
On Wednesday, he discussed marks on the cricket bat and a bent steel plate found in the bathroom door.
"From the forensic evidence, he was on his stumps," Vermeulen said.
Defense attorney Barry Roux countered by suggesting that even with his prosthetic legs on, Pistorius would not be swinging a bat at the same height as an able-bodied person.
The door will also likely be used in court to show the trajectory of the bullets fired at Steenkamp, which the two sides will use to argue whether it was premeditated murder or not.
The court heard Wednesday that the door was intact before the shots were fired.
Under cross-examination, Vermeulen said the first and only time he went to the crime scene was on March 8, 2013, nearly a month after the shooting. Asked about a mark on the door which he did not investigate, he said it was not from a cricket bat.
Roux asserted it was the mark of a prosthetic leg kicking the door with a sock on and that fabric remained in the mark on the door. Vermeulen was unwilling to accept that that was the only possible explanation.
Vermeulen told the court there were footprints consistent with police shoes on the bathroom door at one point and photos of this were shown.
The footprints are not on the door now and Vermeulen said he did not know why. He said that from photos, he knew the door was removed from its hinges and left on the floor for some time.
When Roux suggested they were wiped off by someone who did not know the importance of police procedure, Vermeulen chuckled.
Close-up photographs of the damaged door were shown in court, as well as scuffs on the autographed cricket bat, prompting a Tweet from former South African test cricketer Herschelle Gibbs: "Just saw my signature on the bat used by the accused in Oscar trial...lol".
Pistorius, who took notes and drew sketches during the testimony Wednesday, covered his eyes with his hand when a photo of a panel of the blood-spattered door was shown.
Gun at his bedside
The case against Pistorius is largely circumstantial, prosecutor Gerrie Nel said in his opening statement last week. 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.
Defense attorney Roux has gone after holes, doubts, discrepancies and inconsistencies in prosecution witness stories.
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.
He said he took the gun and fired while calling for Steenkamp to call police. When she didn't answer, he realized it could have been her in the bathroom, he said.
Former girlfriend testifies
Samantha Taylor, a former girlfriend of Pistorius, testified Friday that he reacted similarly once when she was sleeping at his house.
She said he once heard something hit a bathroom window and woke her up to ask if she'd heard it, too, before taking his gun and going to investigate. Taylor said Pistorius woke her up other times when he thought he'd heard a noise.
She also testified that Pistorius slept with a pistol on his bedside table or on the floor beside his prosthetic legs.
Prosecutors appear to have been trying to demonstrate that Pistorius and Steenkamp had a loud argument before the shooting, suggesting it's the reason he killed her.
Who was screaming?
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.
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.
One prosecution witness, the pathologist who conducted the autopsy on Steenkamp, cast doubt on the defense timeline by saying she had probably eaten about two hours before she died. Pistorius says the couple had gone to bed hours before that.
Judge Thokozile Masipa will decide the verdict. 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 gun indictment 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 expected to take at least three weeks.
CNN's Richard Allen Greene reported and wrote from Pretoria, Faith Karimi and Emily Smith wrote from Atlanta. Marie-Louise Gumuchian in London contributed to this report.
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