Key parts of Oscar Pistorius' defense

PRETORIA, South Africa (AP) The defense in Oscar Pistorius' trial will open its case Friday and may first put the double-amputee Olympian accused of murder on the witness stand.

Pistorius claims he fatally shot girlfriend Reeva Steenkamp by mistake thinking she was an intruder in his home. But because he has admitted to killing her, legal experts say Pistorius needs to explain why in court.

Some key parts of the defense's case:

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PISTORIUS' TESTIMONY

Pistorius testifying will be crucial to his defense against the murder charge because it allows the judge to determine his credibility. There is no trial by jury in South Africa so Pistorius has the chance to convince Judge Thokozile Masipa and her two assessors that he did not intend to kill his girlfriend, and is not guilty of murder.

However, Pistorius testifying opens him up to cross-examination by prosecutor Gerrie Nel, who will likely ask the world-famous athlete uncomfortable questions about why he shot four times from close range through a locked toilet door, hitting Steenkamp in the hip, right arm and head, killing her. Prosecutors say Pistorius argued with Steenkamp before he shot her with his 9 mm pistol.

Asked if Pistorius would take the stand, defense lawyer Brian Webber said: "I don't think we have a choice, it's a question of when."

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SCREAMING

One of the foundations of the prosecution's case is that Steenkamp screamed before and during the gunshots. The prosecution's first witness was a neighbor who testified to hearing a woman screaming on the night Steenkamp died. Several neighbors say they heard a woman's screams that night, and a pathologist and police ballistics expert say it's possible and likely.

Pistorius' defense needs to cast doubt on the screaming because it suggests the couple were fighting and that Pistorius knew where Steenkamp was when he shot her, contrasting his version of events.

The defense says that Pistorius was the only person to scream, sometimes in a high-pitched voice, and will use noise tests to show neighbors are mistaken about a woman's voice. They will also fight the prosecution's claim that Steenkamp screamed during the shots through ballistic evidence. Pistorius fired four times with two quick "double-tap" bursts, chief defense lawyer Barry Roux says, and Steenkamp didn't have time to yell out.

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THE DOOR MARKS

Defense experts say police forensic investigators missed two marks on the toilet door through which Pistorius fired the fatal shots. Defense says one is a mark from one of Pistorius' prosthetic legs and the other is from a cricket bat. Although those marks may not deal directly with the killing, defense lawyers will use them to attempt to show that Pistorius' story is true that he tried to kick down the locked door with his prosthetics and then battered it with the bat to try to help Steenkamp. Through it, the defense will argue that Pistorius' entire story is consistent and truthful.

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CHARACTER QUESTIONS

At the start of the trial, the defense objected to part of the prosecution's evidence against Pistorius on the grounds that it was character assassination. The evidence in question largely related to the two firearm charges Pistorius also faces for shooting guns in public. If the defense can show Pistorius was not at fault for the two incidents and he has pleaded not guilty to both then maybe it can start breaking down some of the prosecution's damaging suggestions that Pistorius was angry and trigger happy.

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THE UNEXPLAINED

The defense may discuss so far unexplained pieces of evidence introduced by the prosecution. Prosecutors have shown through crime scene photographs that there was damage to Pistorius' bedroom door, to bathroom tiles and to a metal panel in the bathroom where Steenkamp was shot, inferring that there was a fight and that is why he killed her.

Also, the pathologist who performed the autopsy on Steenkamp's body says she ate no more than two hours before she died, so not before 1 a.m. Pistorius' story is that they were both in the bedroom and ready for bed at 10 p.m. and does not match the pathologist's finding. Pistorius may have to explain those things in his testimony and his defense may have to offer alternative explanations.

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Gerald Imray is on Twitter at www.twitter.com/GeraldImrayAP