Pretoria, South Africa (CNN) -- Track star Oscar Pistorius broke down in court Thursday, the fourth day of his murder trial, as a neighbor described the grisly scenes when he tried to save the athlete's girlfriend after a fatal shooting on Valentine's Day 2013.
The amputee sprinter, 27, nicknamed "Blade Runner," has pleaded not guilty to all four counts against him, saying the killing of model Reeva Steenkamp, 29, was a tragic error and he mistook her for an intruder.
In graphic testimony, witness Johan Stipp, a doctor who lived close by, said he went to Pistorius' residence after hearing shots fired. He said he saw Steenkamp mortally wounded, her brain tissue mixed with blood and Pistorius praying for her to live.
"I remember the first thing he said when I got there was, 'I shot her, I thought she was a burglar and I shot her,' " Stipp told the court in Pretoria.
He said he had arrived and found Steenkamp's body lying at the bottom of the stairs. Pistorius was bent over her body with his left hand over her right groin and two fingers of his right hand in her mouth as he tried to clear her airway.
Stipp tried to assist and said he found no signs of pulse in her neck, no peripheral pulse nor breathing movements.
As the doctor described the details of her injuries to the court, Pistorius broke down with his head in his hands. He was also seen convulsing but then recovered and wiped his face and nose with a handkerchief.
Stipp, who said he had trained in the army with assault rifles and 9 mm pistols -- the gun that killed Steenkamp -- said he had been surprised to hear no ambulance had been called when he arrived. He left after it did.
Lawyers are battling over whether the world-famous athlete killed his girlfriend on purpose or by mistake when he fired four bullets through a closed bathroom door at her.
'He wanted her to live'
Stipp told the court he did not realize Pistorius lived in the house until the day after the shooting when his wife told him.
The doctor was later cross-examined by defense attorney Barry Roux, who has been working toward establishing that Pistorius was the one heard screaming after the shots were fired.
Roux said he had consulted three specialists and the autopsy and asked Stipp: "That person after the shots would not have been able to scream. That person would be nonresponsive, does that make sense to you as a medical doctor?"
"It does," Stipp replied.
The prosecution interjected, saying Steenkamp could have screamed after the first shot. Earlier in the week, prosecutor Gerrie Nel, reading a report from an expert, told the court that of the four bullets fired toward Steenkamp "the fourth bullet hit her in the head. She then died."
Pistorius sat impassively during the exchange between Roux and Stipp. He leaned forward, his head down in his hand, as his attorney asked Stipp if the athlete had wanted Steenkamp to live.
"He definitely wanted her to live, yes," Stipp replied. "He looked sincere to me. He was crying; there were tears on his face."
After the court adjourned for the day Thursday, Pistorius was in tears as his sister consoled him.
Witness grilled over notes
Earlier Roux had cross-examined another neighbor who testified that he heard shouting from Pistorius' house before the shooting.
Charl Johnson, husband of the trial's first witness, was back on the stand after the defense team wanted to retrieve notes he had taken in the weeks after the shooting. Roux grilled Johnson about the notes such as "the screams did not sound like fighting but more like the panic and distress calls of being attacked."
Roux established that Johnson and his wife, Michelle Burger, thought they were hearing noise from a house break-in and not from an act of domestic violence. Nel said Roux was taking individual sentences out of context.
Johnson also testified he owns the same caliber gun as the one that killed Steenkamp and he has fired a firearm before and knows what it sounds like. When Roux asked if he has heard it from the distance of one house to another, he said did not have a similar experience to relate it to but was convinced he knew what the noise sounds like regardless of distance.
Defense pokes holes
The prosecution has put up several witnesses who live near Pistorius and say they heard shouting before the four gunshots that night, while his defense attorney has grilled them relentlessly to raise doubts about their version of events.
Roux on Wednesday said he had no doubt that Johnson believed what he was saying -- but that whether what he was saying was true was a different question.
The defense only has to show that there is reasonable doubt Pistorius intentionally killed Steenkamp.
Judge Thokozile Matilda Masipa will decide whether Pistorius is innocent or guilty. South Africa does not have jury trials.
Johnson and his wife have testified to hearing shouts from Pistorius' house before a series of gunshots around 3 a.m. Johnson told investigators he and his wife didn't discuss their versions of events with each other. But on the stand, Johnson said he did discuss how many shots he and his wife heard and the timing of those shots.
"You and your wife could just as well have stood together in the witness box," Roux remarked. For witnesses to be considered reliable, they have to maintain independent versions of events, Roux said. "You failed, Mr. Johnson," he said.
The lawyer then questioned Johnson on whether what he heard that night was indeed gunshots. Roux said the sounds could have been Pistorius trying to break down the bathroom door with a cricket bat.
Pleading not guilty
Lawyers had briefly detoured Wednesday into another of the four charges Pistorius faces, related to the discharge of a gun he was holding in a crowded restaurant in Johannesburg about a month before the killing.
Pistorius pleaded not guilty Monday to one charge of murder and a firearms charge associated with Steenkamp's killing as well as the two gun indictments unrelated to her death.
The trial is expected to take at least three weeks.
In South Africa, premeditated murder carries a mandatory life sentence, with a minimum of 25 years. Pistorius also could get five years for each gun indictment and 15 years for the firearms charge.
If he isn't convicted of premeditated murder, the sprinter could face a lesser charge of culpable homicide, a crime based on negligence, and could be looking at up to 15 years on that charge, experts said.
Picture of a dream couple
Pistorius and Steenkamp were a young, attractive, high-profile couple popular in South Africa's social circles.
Pistorius, whose "Blade Runner" nickname reflects the special prostheses he uses while running, won six Paralympic gold medals and became the first double-amputee runner to compete in the Olympics, in London in 2012.
Cover girl Steenkamp, who was soon to star in a TV reality show, was on the cusp of becoming a celebrity in her own right. But on February 14, 2013, Steenkamp lay lifeless in a pool of blood on the floor of her boyfriend's house in an upscale gated community in Pretoria.
Moments before, Pistorius said, he had pointed his 9 mm pistol toward an upstairs toilet room and fired four bullets through the locked door.
In court documents, Pistorius has said he heard a noise from the bathroom in the middle of the night and -- feeling vulnerable without his prosthetic legs on -- charged toward the bathroom on his stumps. He has said he shot through the toilet door to protect himself and Steenkamp.
"I felt a sense of terror rushing over me," he said in his court affidavit. "There are no burglar bars across the bathroom window, and I knew that contractors who worked at my house had left the ladders outside.
"It filled me with horror and fear of an intruder or intruders being inside the toilet. I thought he or they must have entered through the unprotected window. As I did not have my prosthetic legs on and felt extremely vulnerable, I knew I had to protect Reeva and myself."
Prosecutors are painting a different picture. They say the pair had an argument and that Steenkamp locked herself in the toilet.
At last year's bail hearing, the state said Pistorius put on his prosthetic legs, collected his gun from under the bed and walked down the hall leading from the bedroom to the bathroom before unloading a flurry of shots through the door.
Pistorius is not claiming self-defense; he is claiming to have been mistaken about his need for self-defense. He is denying he intentionally, unlawfully killed Steenkamp. He has never denied killing her.
The case has put the spotlight on South Africa's rampant gun violence and high crime rates.
Roughly 45 people are murdered every day, according to police statistics, and the number of home burglaries is up 70% in the last decade.
In 2012, more than half of South Africans told the country's police force they were afraid of having their homes broken into. In his affidavit, Pistorius said he had been the victim of violence and burglaries before, including death threats.
CNN's Nic Robertson, Emily Smith, Robyn Curnow, Nick Thompson, Ashley Fantz and Susannah Cullinane contributed to this report.