PRETORIA, South Africa (AP) — Kneeling, a South African police officer on Wednesday swung a cricket bat at a toilet door erected in the courtroom at Oscar Pistorius’ murder trial, using two key pieces of evidence to partially re-enact the night the athlete killed his girlfriend by shooting her through the same door more than a year ago.

Then police Colonel 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, had made glaring missteps in his analysis.

Pistorius’ lawyers successfully secured bail for him last year after contending that police contaminated or tampered with evidence from the scene where he fatally shot Reeva Steenkamp, and they are honing those arguments in the trial.

The scrutiny of the police follows damaging testimony about alleged gunplay by the double-amputee runner in the months before he killed 29-year-old Steenkamp, firing through a closed toilet door in his home before dawn on February 14, 2013. He says the killing was a mistake; the prosecution alleges Pistorius, 27, intentionally shot her after an argument.

The toilet door with four bullet holes and Pistorius’ cricket bat were on display in the courtroom Wednesday. Pistorius says he used the bat to break down the toilet door after realizing that he had made a devastating mistake and shot Steenkamp. In a partial re-enactment, Vermeulen wielded the bat in front of the door to show the position from which Pistorius may have struck the door.

He said he thought Pistorius had struck the door with the bat from a low angle, indicating he was on his stumps at the time. Defence lawyer Barry Roux insisted Pistorius was wearing his prosthetic legs when hitting the door, and the bat marks on the door were lower 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 is important because it could match parts of his story that he accidentally shot Steenkamp, or expose inconsistencies in it. Lawyers have not yet commented on the broader significance of the issue.

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. In a reversal, prosecutor Gerrie Nel said in court Wednesday that he did not dispute the defence’s contention that the runner was indeed on his stumps at the time.

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 toilet door, and battered it with a cricket bat in a panicked attempt to reach his girlfriend.

Vermeulen testified about an unexplained piece of evidence that had not previously been reported in the media. A photograph showed a metal panel on the wall of the main bathroom in Pistorius’ home, away from the toilet cubicle, that had been damaged by what he said was a “hard” object.

Roux turned up the pressure on Vermeulen, saying he had failed to properly look at another mark on the door, a mark Roux says was made by one of Pistorius’ prosthetic legs as he tried to kick down the door and which would be identified by a piece of sock fabric that remained lodged in the wood.

Roux pounced when Vermeulen acknowledged that he finished his study of the door months ago, but wasn’t aware of Pistorius’ version of events until he scanned it within the last week. At one point, the police colonel said he hadn’t “bothered” to reopen his investigation.

“But why not, colonel?” Roux asked. “Or is the approach, rather, ‘I’ve done my investigation and that’s it?'”

Vermeulen said he had sought to be objective in his investigation, and that looking at Pistorius’ version could colour his view of what had happened.

“We are just as much in the service of the accused as the defence is,” he said.

The defence lawyer also indicated that police investigators might have left a shoe print on part of 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 also be seen.

“It looks like a police shoe print,” Vermeulen said.

“How did that come about?” Roux asked. The colonel said he did not know.

The lawyer also asked for a photograph to be shown with a partially obscured Vermeulen examining the door in a forensic room. The pieces of wood the colonel said he hadn’t been able to locate in his investigation were seen just behind the door in the photo.

Vermeulen said it was only obvious to him then, standing in the courtroom in Pretoria around a year later, that the wood fragments he had searched for were right next to him at the time.

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-fibre blades and is a multiple Paralympic medallist. He also competed at the London Olympics but didn’t win a medal.