• She also questioned the reliability of witnesses who say they heard screams and gunshots the night Ms Steenkamp was killed.
• He still faces three other unrelated charges - two for firing a gun in a public space, and one for illegally possessing ammunition.
• The court has been adjourned until 9.30am tomorrow local time (7.30pm tonight NZ time)
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Oscar Pistorius has been sensationally cleared of murdering his girlfriend Reeva Steenkamp - but he must wait a nerve-wracking 24 hours before he discovers if he's guilty of manslaughter.
The South African athlete had broke down in tears in the dock as the judge found him not guilty of the most serious charges against him.
But he was left dangling when the judge abruptly broke off her verdict just as she was addressing the question of culpable homicide (manslaughter). And her last comments before she adjourned indicated she was inclined to find Pistorius guilty.
Pistorius had been accused of shooting the 30-year-old model during a furious row at his Pretoria home on Valentine's Day last year. But he had always maintained - through cries, wails and breakdowns during his six-week trial - that he shot the law graduate in self-defence after mistaking her for an intruder.
Judge Thokozile Masipa ruled that there were not enough facts to prove the charge of pre-meditated murder against the athlete.
Although she described the 27-year-old as a "very poor" and "evasive" witness, she said this did not mean the track star was necessary guilty in a case that she said was based entirely on circumstantial evidence.
"The state has not proved beyond reasonable doubt that the accused is guilty of premeditated murder," she told the Pretoria High Court. "There are just not enough facts to support such a finding."
She also found him not guilty of murder without premeditation (dolus eventualis). She said the prosecution had not proved that Pistorius had foreseen he would have killed the person behind the toilet door when he opened fire.
However, "there is no doubt that when the accused fired he acted unlawfully - there was no intruder", she said.
Before she adjourned she said that Pistorius had acted "too hastily and used excessive force ... It is clear his conduct was negligent."
He could have called for help, she said, instead of going into the bathroom with a loaded weapon. "A reasonable person would not have fired four shots into the toilet cubicle," she said. A reasonable person, she said, would have foreseen that the person inside could have died.
She we will not resume her verdict until Friday evening New Zealand time. If found guilty of culpable homicide, Pistorius could face up to 15 years in prison
Oscar Pistorius and Reeva Steenkamp in September, 2012. Photo / Getty Images
Oscar Pistorius and Reeva Steenkamp in Johannesburg on November 4, 2012. Photo / Getty Images
Judge Masipa had earlier discredited large chunks of the prosecution case against the athlete.
During her summary of the case, she dismissed a series of heated text messages between the couple which the prosecution claimed was evidence they were in a volatile relationship.
Prosecutor Gerrie Nel had used them in an attempt to bolster his claim that the athlete shot his girlfriend in a fit of rage.
But Judge Masipa said that inference could not be made, adding: "Nothing of this proves anything at all."
Oscar Pistorius celebrates as he wins gold in the Men's 400m at the London 2012 Paralympic Games. Photo / Getty Images
Oscar Pistorius during the medal ceremony for the Men's 400m at the London 2012 Paralympic Games. Photo / Getty Images
She also said there was "some doubt" that a woman screamed on the night because of contradictory witness testimony, which she believed had been contaminated by media reports.
That finding cast serious doubt on another central tenet of the prosecution case that the couple were having a furious row when he shot the model.
Pistorius had arrived at the courthouse at 8am local time in a dark suit and white shirt. He was surrounded by a ring of bodyguards and police who escorted him through a scrum of reporters, supporters and television cameras.
He appeared calm, staring straight ahead and making no comment.
Shortly after, his estranged father Henke arrived, putting in only his second appearance at the trial that has captivated a global audience for the last six months.
"I'm just here to support him," Henke told reporters.
The athlete's brother, Carl Pistorius, arrived at the courthouse in a wheelchair with both his legs in splints in his first appearance since being seriously injured inn a car crash earlier this year.
As Judge Masipa called Pretoria's High Court to order, Pistorius appeared tense. He sat throughout the judgement with a sick bucket by his side in the dock.
Oscar Pistorius during the trial at the Pretoria High Court. Photo / Getty Images
Reeva Steenkamp's mother, June Steenkamp, listens to evidence. Photo / Getty Images
Pistorius had denied all the charges, including three other firearm-related counts.
Prosecutor Gerrie Nel took 15 days to lay out his case back in March, arguing that Pistorius deliberately killed Ms Steenkamp by firing four rounds from a 9mm pistol through a closed toilet door.
The personality of the Paralympian gold medallist, who won worldwide fame when he competed on his prosthetic "blades" against able-bodied runners at the London Olympics, was a focus of the trial.
Prosecutors had described him as an egotistical liar obsessed with guns, fast cars and beautiful women, who was not prepared to take responsibility for his actions.
Defence lawyers had said that there were "two Oscars" - a world-class athlete and a highly vulnerable individual with a serious disability who acted out of fear, not anger, when he fired the fatal shots.
Defence lawyer Barry Roux said during his own wrapping-up that psychological evidence had proven the track star had a heightened fight response because of his disability and was in a terrified and vulnerable state when he shot Steenkamp.
"You're standing at that door. You're vulnerable. You're anxious. You're trained as an athlete to react. Take all those factors into account," Roux said, adding that Pistorius had felt exposed because he was standing on the stumps of his legs.
Mr Nel had responded to those claims by insisting that Pistorius undergo psychiatric tests to establish if he was mentally well enough to stand trial. But, after a court-ordered 30-day assessment, experts ruled that he was capable of understanding the wrongfulness of his actions when he fired the fatal shots.
The trial's tensest and most dramatic moments came in several days of highly charged testimony from Pistorius.
His voice thick with emotion, the athlete began his evidence by saying sorry to Ms Steenkamp's family.
Watched by the model's mother June, he said: "I was simply trying to protect Reeva. I can promise that when she went to bed that night she felt loved."
Mrs Steenkamp later told Hello! Magazine she has forgiven him. "I don't hate Oscar," she said.
Pistorius went on to describe how he had suffered from nightmares and sleeplessness following the incident, while also recounting the impact on him of previous instances of crime.
Later in his testimony, the court had to adjourn as Pistorius broke down sobbing and howling while describing the aftermath of the shooting.'I sat over Reeva and I cried," he said.
As in the famed live television trial of US football hero OJ Simpson, who was accused of murdering his wife 20 years ago, most viewers seemed to have already decided on Pistorius's guilt or innocence before the verdict.
But unlike the Simpson trial, in which he was controversially acquitted by a jury, Masipa is assisted only by two assessors.
- additional reporting Independent