Testifying on the fourth day of Pistorius' trial, neighbour Johan Stipp said he entered the athlete's home last year a few minutes after hearing screams and shots to find the distraught sprinter kneeling over the lifeless body of a woman.
"'I shot her. I thought she was a burglar and I shot her,'" Stipp quoted the 27-year-old Pistorius as saying.
Stipp, a doctor, went on to describe his futile attempts to revive Steenkamp, a 29-year-old model and law graduate who been dating Pistorius for a few months.
She died after being hit by three rounds, including one to the head, out of four fired by Pistorius through the locked door of an upstairs toilet. He denies murder, arguing that it was a tragic mistake and that he mistook her for an intruder.
As Stipp checked Steenkamp for signs of life, Pistorius was begging him to save her life, Stipp told the court.
"Oscar was crying all the time. He prayed to God: 'Please let her live, she must not die,'" he said.
At one point, when Pistorius left Steenkamp, Stipp and housing complex manager Johan Stander to go upstairs, Stipp thought he might be about to kill himself.
"I noticed that Oscar was going upstairs and I asked Mr. Stander if he knew where the gun was because it was obvious that Oscar was emotionally very, very upset," he said.
"I didn't know the situation in the house so I thought maybe he was going to hurt himself."
Pistorius had his lower legs amputated as baby but went on to achieve international fame as the "fastest man on no legs", running on carbon-fibre prosthetic limbs.
Already one of the best-known Paralympic athletes, he ascended to the pantheon of track greats at the 2012 London Olympics when he reached the 400 metres semi-final competing against able-bodied athletes.
With rugged good looks and an easy smile, he was a sponsors' dream, but since he shot Steenkamp on Valentine's Day last year a different side to his carefully groomed media persona has emerged.
On Wednesday, the court heard how Pistorius had accidentally fired a pistol under the table in a packed restaurant - right next to a child - and then asked a friend to take the blame.
The prosecution has used the incident to try to portray an image of Pistorius as a gun-obsessed hot-head. If found guilty of intentional murder, Pistorius is likely to spend at least 25 years behind bars.
Other witnesses have testified to hearing a woman's terrified screams before and during the volley of shots.
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