Police Col. J.G. Vermeulen faced tough questioning from Pistorius' defence lawyer, who tried to discredit the police investigation of the shooting and alleged that Vermeulen, a forensic expert, made glaring missteps in his analysis.
Pistorius' lawyers secured bail for him last year after contending that police contaminated or tampered with evidence from the home where he fatally shot Reeva Steenkamp, firing through the bathroom door as she cowered inside during the pre-dawn hours of Valentine's Day 2013.
Pistorius says the killing was a mistake; the prosecution alleges the 27-year-old double-amputee intentionally shot Steenkamp after an argument.
The actual door that Pistorius shot through a year ago was erected in the Pretoria courtroom Wednesday, and the bat he used that night was used in the demonstrations. There was even a toilet cubicle behind the door, recreated to the exact specifications of the small area of Pistorius' bathroom where the 29-year-old model was fatally shot, Vermeulen said.
Below the door's handle, four bullet holes were clearly visible. Steenkamp was hit three times in the hip, arm and head. One shot missed, the court has heard.
Pistorius says he used the bat to break down the door after realizing he shot Steenkamp, mistaking her for an intruder.
Kneeling, Vermeulen swung the bat to show how he believed Pistorius struck the door from a low angle, indicating he was on his stumps at the time. Defence lawyer Barry Roux insisted Pistorius was wearing his prosthetic legs, saying the bat marks on the door were low because he swung with a bent back.
The back-and-forth over whether Pistorius, the first amputee to run at the Olympics, was on his prosthetic limbs or not is important because it could match parts of his story that he accidentally shot Steenkamp, or expose inconsistencies in it.
Last year, prosecutors maintained Pistorius was on his prostheses when he fired through the door, arguing the runner planned the killing while putting on his artificial limbs. But in a reversal, prosecutor Gerrie Nel said in court Wednesday that he did not dispute the defence's contention that the runner was on his stumps when he opened fire.
The athlete has said he fearfully approached the bathroom on his stumps and shot Steenkamp by mistake, thinking she was an intruder hiding behind the door. According to his account, he then put on his prostheses and tried to kick down the locked door, striking it with a cricket bat in a panicked attempt to reach his girlfriend.
Roux turned up the pressure on Vermeulen, saying the police investigator failed to properly look at another mark on the door the defence says was made by Pistorius' prosthetic leg as he tried to kick it down, leaving a piece of sock fabric lodged in the wood.
The defence lawyer also indicated that police investigators might have left a shoe print on the door, the door itself might have been kept in a policeman's office and not in proper evidence storage, and missing fragments from the door weren't examined. Vermeulen said the door was kept in a "body bag."
In an evidence photo, a faint footmark on one of the door panels could be seen.
"It looks like a police shoe print," Vermeulen said.
"How did that come about?" Roux asked. Vermeulen said he did not know.
Earlier, Vermeulen also said a metal panel on the wall of the main bathroom in Pistorius' home had been damaged by being hit with a "hard" object. The panel was new evidence. A photo of the damaged plate was shown.
Pistorius faces a possible life sentence if convicted of murder for killing Steenkamp. The judge, who watched the demonstrations Wednesday, will ultimately decide on the verdict. There is no trial by jury in South Africa.
Pistorius was born without fibula bones because of a congenital defect, and his legs were amputated when he was 11 months old. He ran on carbon-fiber blades and is a multiple Paralympic medallist . He also competed at the London Olympics but didn't win a medal.
Torchia reported from Johannesburg.