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Warrant officer Hilton Botha, a detective with 24 years on the force, also told the Pretoria magistrates court in a bail hearing that police had found two containers of testosterone and needles in Pistorius' bedroom.
The athlete's defence team said the containers held a herbal remedy taken legally by many athletes, landed some blows to Botha's portrayal of a cold-blooded killing and described inconsistencies and errors in the initial investigation.
Pistorius, a double amputee dubbed "Blade Runner" because of his carbon fibre racing blades, sobbed uncontrollably as Botha presented his testimony about the death of Reeva Steenkamp, 29.
The law graduate and model was in the toilet of the athlete's home when she was shot dead in the early hours of Feb. 14 - Valentine's Day. She was hit in the head, arm and hip.
The shooting and allegations that have emerged at the hearing have stunned South Africa and millions of people around the world who regarded Pistorius, who has no lower legs, as the epitome of physical triumph over adversity.
"One of our witnesses heard a fight, two people talking loudly at each other ... from two in the morning to three," Botha told the court. Pistorius' first call after the incident was to the manager of his high-security complex at 3.19 am, Botha said.
In an affidavit delivered on Tuesday, Pistorius said he woke in the middle of the night and thought an intruder had climbed through his bathroom window and entered the adjoining toilet.
The 26-year-old said he grabbed a 9-mm pistol from under his bed and went into the bathroom.
Pistorius - the highest-profile athlete in the history of the Paralympics - then described how he fired into the locked toilet door in a blind panic in the mistaken belief that the intruder was lurking inside.
After four hours of testimony, the hearing was adjourned until Thursday. It is expected to conclude this week, once the defence and prosecution have outlined their central arguments.
It may then be several months before a trial. If convicted of premeditated murder, Pistorius faces life in jail.
His uncle Arnold said Pistorius was in "extreme shock" and had only started eating again on Tuesday evening. He had spent some of his five days in police custody reading the Bible, he added.
Botha, who arrived on the scene an hour after the shooting, challenged Pistorius' affidavit.
"I believe he knew she was in the bathroom and he shot four shots through the door," the detective said, adding that the angle at which the rounds were fired suggested they were aimed at somebody on the toilet.
Pistorius had said he moved into the bathroom on his stumps - the reason he felt so vulnerable - but Botha said the shots went in a "top to bottom" trajectory, suggesting Pistorius was wearing his artificial legs when he pulled the trigger.
"It seems to me it was fired down," he said.
One of the spent rounds was recovered from the toilet bowl, Botha said.
He also cited a witness in the upscale gated community near Pretoria where Pistorius lived as saying he heard a shot, followed 17 minutes later by more shots. Another witness spoke of a shot, followed by screams, then more shots, he said.
After a vigorous cross-examination from Pistorius' defence, a rattled Botha halved his original estimate of the distance between the witnesses and Pistorius' home to 300 metres.
Lead defence counsel Barry Roux challenged Botha, describing oversights and slip-ups in the initial police investigation.
He disputed the policeman's reference to "testosterone", saying the substance found in Pistorius' bedroom was a legitimate herbal remedy called "testo-composutim co-enzyme".
Details on the makeup of testo-composutim co-enzyme were not immediately available.
Administering testosterone as an anabolic agent is banned at all times under World Anti-Doping Agency rules for sports people.
"HE WILL BOUNCE BACK"
The arrest of Pistorius stunned millions who had watched in awe last year as the Olympic and Paralympic sprinter reached the semi-final of the 400 metres in the London Olympics.
But the impact has been greatest in sports-mad South Africa, where Pistorius was seen as a rare hero who commanded respect from both blacks and whites, transcending the racial divides that persist 19 years after the end of apartheid.
The case has drawn further attention to endemic violence against women in the country after the gang rape, mutilation and murder of a 17-year-old near Cape Town this month.
Arnold Pistorius accused the state of launching a 'public trial' by leaking evidence to the media, and reiterated his belief that his nephew would be exonerated and return to the track and public life.
"Truth will prevail," he said in an interview with South Africa's ENCA television channel. "I know the truth. I know exactly what happened. The puzzle is not going to fit where the state wants to fit it."
"He will bounce back and be greater than ever."
The sprinter's sponsors do not share that confidence.
Sportswear giant Nike and French designer Thierry Mugler have both dropped him from future advertising campaigns and cosmetics firm Clarins is recalling a male perfume range bearing Pistorius' signature.
His other sponsors include British telecoms firm BT and sunglasses maker Oakley.