Detective Hilton Botha, who gave evidence against the Paralympian yesterday, took to the witness box again today, despite revelations that he is facing court himself over charges of attempted murder.
A top South African detective will now take over the investigation.
The change was announced after a third day of testimony at the Pistorius's bail hearing.
It came as Nike today confirmed it had suspended its contract with Pistorius.
In a statement, the company said: "We believe Oscar Pistorius should be afforded due process and we will continue to monitor the situation closely."
Botha returned to the witness box today to be questioned by magistrate Desmond Nair. He is due to appear in court himself in May, with two other police officers, accused of firing shots while trying to stop a minibus in an incident believed to have happened in October 2011.
Police Brigadier Neville Malila, of the South African Police Service, said officers learned yesterday that the charges against Botha and the two others, which had previously been dropped, had been reinstated by the Director of Public Prosecutions.
He told Sky News the seven counts of attempted murder, dating back to 2011, only came to light late yesterday afternoon and relate to seven people in the minibus.
Asked to explain the charges, Botha told the court the case was dropped, he was not drunk and was chasing suspects.
"Blade Runner" Pistorius was in court today for the third day of his bail hearing.
The sports star has admitted shooting 29-year-old model Reeva Steenkamp, thinking she was a burglar or burglars, who were in the bathroom. He said he opened fire in the dark because he was too scared to turn on a light.
Realising his mistake, he broke the door down with a cricket bat and carried her downstairs, he said.
Yesterday Botha acknowledged the prosecution had no evidence challenging the double-amputee Paralympian's claim that he killed his girlfriend accidentally, as well as admitting a number of police blunders in the investigation.
But he said there was "no way" he believed Pistorius's version of events, adding: "I believe he knew she was in the bathroom and he shot four shots through the door."
Today the detective admitted he had not yet obtained telephone or financial records for Steenkamp, and after being told by Mr Nair that there appeared to have been a lack of urgency in getting them, admitted it should have been done.
Prosecutor Gerrie Nel referred to a magazine interview with Pistorius in an Afrikaans magazine which referred to him being given a house in Italy, but the Paralympian's defence advocate Barry Roux said he does not own any property abroad.
Mr Roux said the onus was on the state to show that it was in the interests of justice for Pistorius to remain in prison, saying: "The evidence does not even show that the applicant committed a murder.
"The poor quality of the evidence presented by chief investigating officer Botha exposed disastrous shortcomings in the state's case."
The court was cleared briefly after magistrate Mr Nair said there was a "threat" outside but proceedings then resumed.
Pistorius sobbed again in court as defence advocate Barry Roux summed up his case.
He claimed Detective Botha had been selective with what he said, and seemed determined to "bolster the state's case".
He repeated assertions made yesterday that Mr Botha could not refute Pistorius's version of what happened.
He said Steenkamp staying with the Paralympian was consistent with a loving relationship, and that after she was shot, the evidence suggested Pistorius was desperate to save her life.
He said the fact there was no urine in Steenkamp's bladder showed she had got up to go to the toilet.