Oscar Pistorius's lawyers begin repairing their defense today in his trial for the killing of his girlfriend Reeva Steenkamp after its lead expert witness stumbled and the Paralympic athlete appeared to change his original self-defense plea.

Defense lawyer Barry Roux will continue calling witnesses to back the athlete's testimony that he feared for his life when he shot through the toilet cubicle door at his home on Valentine's Day last year and that he didn't know Steenkamp was not in the bed at the time because the room was too dark.

Before the trial at the high court in the capital, Pretoria, was adjourned on April 17, former police investigator Roger Dixon was repeatedly challenged by Prosecutor Gerrie Nel about his qualifications to testify about the crime scene. A professor of geology, he never physically touched evidence he gave an opinion on, and he contradicted Pistorius's version of the way Steenkamp had fallen after being shot.

Double-amputee Pistorius, who's pleaded not guilty to premeditated murder, spent seven days on the stand, five them questioned by Nel. He ended his testimony on April 14 after appearing to contradict his original claim of self defense when he said that he fired four shots through the door "accidentally." Nel said that would mean Pistorius, 27, was claiming a defense of involuntary action.

"Your version is not only untruthful but it is so improbable that it couldn't have happened," Nel said. "Who should we blame for the black talon round that ripped through her?" he said, referring to the make of hollow-point bullet used in the shooting.

Woman’s Screams

Nel has argued that Pistorius shot Steenkamp after they had an argument, with neighbors testifying they heard shouting and a woman's screams shortly after 3 a.m. on Feb. 14, last year. Pistorius says police tampered with evidence, moving items at the crime scene.

The trial, which started on March 3, is being broadcast live on radio and TV. Proceedings were adjourned until May 5 because of a series of holidays in South Africa. Pistorius has also pleaded not guilty to three gun-related charges.

Judge Thokozile Masipa, who will give the final judgment in the case because South Africa doesn't have a jury system, could consider a lesser charge of culpable homicide if she rules that the act wasn’t premeditated. Pistorius would face a minimum of 25 years in jail if convicted of premeditated murder.

Before his last day of testimony, Dixon expressed his frustration on his Facebook Inc. page: "Third day in court today. Let’s see how much of my credibility, integrity and professional reputation is destroyed. It is difficult to get belief in those who will not listen because it is not what they want to hear. After that, beer!"

Religious Man

Nel has sought to undermine the defense's portrayal of Pistorius as a religious man with a deep fear of crime who was in a loving relationship with Steenkamp. He accused Pistorius of tailoring his testimony to fit in with the evidence.

Known as the Blade Runner because of his J-shaped prosthetic running blades, Pistorius has been free on 1 million rand ($96,000) bail since February last year.

The charges have derailed the running career of the winner of six Paralympic gold medals and cost Pistorius sponsorship deals with Nike Inc., Luxottica Group SpA’s Oakley and Ossur hf, the Icelandic company that manufacturers the blades he uses.

Pistorius was the first double amputee to compete at the Olympic Games in London in 2012.(Bloomberg)