As the girlfriend he shot lay dead or dying in his home, a weeping, praying Oscar Pistorius knelt at her side and struggled in vain to help her breathe by holding two fingers in her clenched mouth, a witness testified Thursday at the double-amputee runner's murder trial.

"'I shot her. I thought she was a burglar. I shot her,"' radiologist Johan Stipp recalled Pistorius saying.

The testimony in high court in Pistorius's murder trial was riveting and was the first detailed public description of the immediate aftermath of the shooting of Steenkamp, a 29-year-old model, by the double-amputee Paralympic champion in the pre-dawn hours of Feb. 14 Valentine's Day last year. Pistorius has pleaded not guilty.

Minutes after arriving at the scene, Stipp said Pistorius (who Stipp said he only figured out later was the famous Olympian) went upstairs — the area Steenkamp was shot — and then returned. At that point, Stipp said he was concerned that the gun used in the shooting had not been recovered and that a distraught Pistorius was going to harm himself. The testimony did not address what Pistorius did when he went upstairs.

South Africa Pistorius Trial

Oscar Pistorius sits in the dock on the fourth day of his trial at Pretoria's high court. (Marco Longari/AP)

On the ground, Steenkamp had suffered a devastating head wound, Stipp said.

"It was obvious that she was mortally wounded," Stipp said as he described what he saw at Pistorius' villa.

"I tried to assist her," said Stipp, a trained medical doctor with years of study. "I tried to open an airway."

Sitting on a courtroom bench, Pistorius bent forward and put his hand over his face, then moved them to cover both ears, as Stipp spoke. He stayed that way for a while, even when one of his lawyers reached back and, in a gesture of reassurance, touched him on the head.

Doctor didn’t know distraught man was Pistorius

Stipp recalled arriving at the scene under questioning by prosecutor Gerrie Nel and also fielded questions from Pistorius’ lawyer Barry Roux.

"I went near her and as I bent down, I also noticed a man on the left kneeling by her side," Stipp said of first seeing Pistorius and Steenkamp. "He had his left hand on her right groin, and his right hand, the second and third fingers in her mouth."

Pistorius's version of events

At his bail hearing last year, Pistorius said in a statement read by his lawyer Barry Roux that, after he realized he had shot Steenkamp, he pulled on his prosthetic legs and tried to kick down the toilet door before finally giving up and bashing it in with a cricket bat.

Inside, he said he found Steenkamp, slumped over but still alive. He said he lifted her bloodied body and carried her downstairs to seek medical help.

Pistorius did not testify Thursday.

Stipp, who said he didn't know the man was track star Pistorius until later, said he tried to help but Steenkamp showed no signs of life. Stipp said he noticed a wound in her right thigh, in her upper arm and in the right side of the head, and there was brain tissue around the skull.

"She had no pulse in the neck, she had no peripheral pulse. She had no breathing movements that she made," Stipp said.

Pistorius is charged with shooting Steenkamp three times out of four shots through a toilet door in his home. Prosecutors said the athlete intentionally killed Steenkamp after an argument, but Pistorius says it was a mistake and he thought she was an intruder.

Stipp said Pistorius was distraught after the shooting.

"Oscar was crying all the time," he said. "He was praying to God, 'Please let her live."'

"Oscar said he would dedicate "his life and her life to God" if she would live, he said.

Pistorius defence grills witness

Earlier, Roux continued to grill Pistorius' neighbour Charl Johnson about how many gunshots he thought he heard and at what time on the night Steenkamp's shooting death took place.

Roux said Johnson's testimony and statements to police were manipulated to match those of his wife, who testified on the opening day, and were an attempt to "incriminate the accused."

"I can confidently say I heard gunshots," Johnson insisted on cross-examination by Roux on the fourth day of Pistorius's murder trial. Later, Johnson said: "I'm convinced that I heard a lady's voice."

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Pistorius's sister Aimee, right, speaks to a member of the Steenkamp family during a break in the trial. (Marco Longari/AFP/Getty)

Roux argues that prosecution witnesses Johnson and his wife are mistaken over what they heard on the night Pistorius killed his girlfriend by firing four times through a toilet door.

Roux says the banging sounds were actually Pistorius hitting a toilet door with a bat and the screaming was the distressed athlete calling for help — and there were no sounds from Steenkamp. Pistorius says he shot Steenkamp by mistake.

Johnson said he "disputed" some of what Roux was saying and described in more detail what he heard on the night Pistorius shot his girlfriend to death. Johnson and his wife live 177 metres from the villa where Steenkamp died.

"The fear … in the lady person's calls contrasted with a very monotone male voice," Johnson testified. "The man almost sounded embarrassed to be calling for help."

Roux did get Johnson to concede right at the end of the court session that he never heard what he thought was the woman's voice and the man's voice at the same time. Roux wants to show that it was the same person — Pistorius — screaming.