Ballistics and Screams Threaten Oscar Pistorius Defense

PRETORIA, South Africa March 27, 2014— -- Oscar Pistorius and his lawyers have several crucial elements of the prosecution's case to overcome when they begin their defense Friday, particularly the ballistics testimony on the sequence of shots and neighbors' testimony on hearing a woman scream.

Pistorius will be the star witness -- and possibly the first witness -- in his defense. His testimony could last days and Pistorius will be a target of prosecutor Gerrie Nel who will try to catch the Blade Runner in a contradiction or an flash of anger.

Pistorius has been emotional while watching the testimony, sticking his fingers in his ears during descriptions of how he shot his girlfriend Reeva Steenkamp through a locked bathroom door before dawn on Valentine's Day 2013, and covering his ears with his hands as her injuries were detailed. Twice he vomited in court when confronted with photos of his slain lover.

Pistorius, who claims he mistook Steenkamp for an intruder, could be sentenced to at least 25 years in prison if convicted.

The case could come down to Pistorius' testimony, legal expert Emma Sadleir told "Good Morning America."

"We are only interested in whether in his own mind he thought he was acting unlawfully in that moment," Saddleir said. "He is the only person who can tell us this."

Pistorius' fate will be decided by a judge acting alone, not a jury.

In convincing the judge that he is innocent, Pistorius will have to also overcome several key points made by the prosecution.

Attorney Anton Smith cited four elements of the prosecution's case that stand out: the ballistics, Pistorius' knowledge of South African gun laws, the pathologist's report and five neighbors who all testified about hearing a woman screaming before the gunshots.

"According to me, the most important witness for the state was ballistic expert Captain Chris Mangena who testified about the succession of shots and where those shots hit," Smith told ABC News. "What is important about this, is that the 'double-tap' theory of the defense was shot down."

Pistorius' legal team have argued that when he fired four bullets through the bathroom door, what people heard was a "double tap," two rounds fired in quick succession.

Using lasers to track the trajectory of the bullets, Mangena told the court that the first shot hit Steenkamp in the hip, knocking her onto a magazine rack. One bullet missed Steenkamp, while one struck her in the right arm above the elbow and the other to the right side of her head.

Mangena's testimony appeared to question Pistorius' double tap claim that because "both shots would have hit her around the waist area, where the first shot was," Smith said.

Smith said Mangena's evidence was supported by Gert Saayman, the pathologist who performed the autopsy on Steenkamp's body. Both experts were of the opinion that the first wound Steenkamp sustained was in the hip, followed by the two other wounds.

The sequence of the shots dovetails with testimony from neighbors who insisted they heard a woman screaming before hearing thuds sounding like gunshots.

"If we can assume he shot the first shot, there was a pause and scream, he should have realized that it was Reeva, or considered that it may have been Reeva," Smith said.

Smith said the evidence from Sean Rens, manager of the International Firearm Training Academy, is also extremely important to the state's case. Rens told the court that Pistorius had ordered several firearms from him and took a competency test in order to be declared fit to possess those guns.