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PRETORIA, South Africa (CNN) -- A police forensic expert testified Wednesday that it appeared Oscar Pistorius was not wearing his prosthetic legs when he knocked down a bathroom door with a cricket bat after killing his girlfriend.
Pistorius has said he tried to break open the door when he realized he'd shot his girlfriend, Reeva Steenkamp, last year.
Forensic scientist Col. JG Vermeulen took the stand to discuss marks on a cricket bat and a bent steel plate found in the bathroom door after the shooting.
In his testimony, which used the actual bathroom door, Vermeulen had to squat to highlight the marks on the door. He said the location of the spots on the door seem consistent with Pistorius not wearing his prosthetics.
The door will also be used to show the trajectory of the bullet, which both sides can use to argue whether it was premeditated murder or not.
Pistorius, the first double-amputee to run in the Olympic Games, regularly wears prosthetic legs. If he was on his stumps at the time, the trajectory will be lower and the defense can argue that he was feeling vulnerable and didn't have time to think.
Pistorius has pleaded not guilty to murder in the shooting on Valentine's Day last year.
He admits killing the 29-year-old by shooting her through a locked bathroom door in his house. However, he says he mistook her for a burglar in the middle of the night, and argues it was a tragic but understandable mistake.
Friend: Pistorius sped, fired
Before the scientist took the stand, a friend of Pistorius who was with him twice when guns went off in the South African track star's hands was back on the stand to testify against him.
Prosecutors trying to convict Pistorius of murdering his girlfriend have charged him with breaking gun laws on both occasions.
The incidents were not connected to the fatal shooting of Steenkamp, but prosecutors appear to be using them to demonstrate that Pistorius is not safe around guns.
He grinned and shook his head as his friend, Darren Fresco, testified Wednesday about speeding during an incident that ended with the track star firing a shot out the sunroof of the car.
Pistorius was driving about 200 kilometers per hour, Fresco said. He said Pistorius was "furious" that police had touched his gun in one of the incidents, and later fired a shot out the sunroof of the car Fresco was driving.
The two men had been pulled over for speeding.
Fresco also testified about an incident at Tashas Restaurant, where Pistorius is accused of asking him to take the blame after he discharged Fresco's pistol under the table a month before Steenkamp's death.
Judge Thokozile Masipa told Fresco he would be immune from prosecution for self-incriminating testimony if he tells the truth.
But at times Tuesday, he seemed evasive, insisting he did not remember what happened after dinner on the night of the sunroof incident, despite having described the day in detail up to that point.
At other times, he got testy with the cross-examination of defense lawyer Barry Roux, saying he did not have a ruler with him when asked how close he and Pistorius were when the gun changed hands at Tashas.
And there were sharp intakes of breath in court toward the end of the day when Fresco let slip that he had been following some of the trial on Twitter.
Fresco had volunteered on the stand that he was not wearing "tracksuit bottoms" at Tashas because it was summer.
Roux pounced, asking who had said anything about tracksuit bottoms.
"People have said to me, 'Your name's been mentioned; your name's been mentioned; your name's been mentioned," Fresco said. "I wanted to find out what was said."
Witnesses are not supposed to listen to the testimony of other witnesses in order to keep their testimony from being tainted.
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 neighbors who heard screaming and bangs that night, experts, current and former friends of Pistorius, and a security guard who sped to the scene because of reports of gunshots.
Defense lawyer Roux has gone after holes, doubts, discrepancies and inconsistencies in prosecution witness stories.
Many prosecution witnesses' stories 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 to be an intruder.
He said he took
the gun and fired while calling for Steenkamp to call the 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.
Masipa will decide Pistorius' 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 Green reported and wrote from Pretoria, and Faith Karimi and Emily Smith from Atlanta.
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