By Ed Cropley
JOHANNESBURG (Reuters) - Eighteen months ago when he granted Oscar Pistorius bail after the killing of his girlfriend, South African magistrate Desmond Nair noted a number of "improbabilities" in the Olympic and Paralympic star's account of the shooting.
After 41 days of testimony and drama in the Pretoria High Court, Pistorius's freedom hangs on whether the prosecution has made its case well enough to convince judge Thokozile Masipa that such improbabilities cannot be "reasonably possibly true".
Pistorius, a double-amputee who made it to the semi-final of the 400 meters at the London 2012 Olympics, says he fired through the door into the toilet cubicle in the mistaken belief he was defending himself from a burglar.
Why, Nair had asked, did Pistorius not find out who was in the toilet before firing four 9mm hollow-point rounds?
And why did Reeva Steenkamp not let him know she was there?
With Pistorius the only direct witness, much of the state's case rests on forensics, including evidence that the sequence of Steenkamp's injuries would have allowed her to cry out, and on the testimony of neighbors who say they heard the terrified screams of a woman immediately before and during a volley of shots.
The defense says the screams came from Pistorius, at an unusually high pitch for a man because of the distress of discovering that he had unwittingly shot Steenkamp.
Prosecutor Gerrie Nel, known as "The Pitbull", painted a picture of Pistorius, 27, as a gun-obsessed hot-head who killed Steenkamp, 29, in a fit of rage after a row in the early hours of Valentine's Day last year.
Pistorius and his representatives have made no comment outside the trial since it started in early March, other than to thank friends, family and supporters.
South Africa's apartheid government scrapped trial by jury in the late 1960s, meaning that 66-year-old Masipa, only the second black woman to rise to the bench, will ultimately decide Pistorius's fate when she delivers her verdict on Sept. 11.
Possible verdicts range from convictions for premeditated murder - and a minimum 25-year sentence - or a lesser count of murder, to culpable homicide - with a maximum 15-year sentence - if Masipa believes he did not intend to kill Steenkamp but did so by firing negligently or recklessly.
She could acquit if she accepts that Pistorius believed he was acting in self-defense - known formally as 'putative private defense' - when he pulled the trigger.
"This case is about the credibility of Oscar Pistorius," said Johannesburg-based advocate Riaan Louw. "If he's not a credible witness and the judge does not accept his testimony, he's going to be convicted on either murder or culpable homicide."
Pistorius gave five days of cross-examination in April under the gaze of the world's media.
By turns calm, tearful and combative, at one point he appeared to confuse the central pillar of his legal defense under questioning from Nel. Having argued that the killing was a deliberate but mistaken act of self-preservation, he then said he pulled the trigger without thinking - an assertion that would match a so-called automaton defense but not self-defense.
Three times when under pressure during cross-examination, Pistorius blamed his legal team for differences between or omissions from the sworn affidavit prepared for his bail deposition and his evidence in court.
"When it starts happening regularly like this, and every time you get a difficult question you blame it on your lawyer, the judge is going to make a very careful note," Johannesburg-based criminal defense advocate Mannie Wits said.
There were other inconsistencies in his evidence, including an admission that he had not after all left the bedroom to get a fan from the balcony - contradicting his bail deposition, in which he said it was during this absence from the bedroom that Steenkamp must have gone to the toilet.
If the judge accepts his testimony and agrees that the shooting was self-defense, she must still decide whether shooting four times at an unidentified target on the other side of a closed door is legally "reasonable".
After the end of apartheid in 1994, South Africa tightened its gun laws, such that a person can shoot only if there is an imminent and direct threat - a principle Pistorius said he had read and understood as part of his firearms license test.
"He knew there was a human being in the toilet. That's his evidence," prosecutor Nel said in closing remarks.
"His intention was to kill a human being. He has fired indiscriminately into that toilet. Then, M'lady, he is guilty of murder. There must be consequences."
(Reporting by Ed Cropley; Editing by Pascal Fletcher and Will Waterman)