JOHANNESBURG (AFP) - South African police assigned a top new detective Thursday to take over the bungled Oscar Pistorius investigation after it emerged the lead officer faced seven charges of attempted murder, dramatically undermining the prosecution case against the sprint star.

"We recognise the significance, the importance... and the severity of the matter," police commissioner Mangwashi 'Riah' Phiyega said as she announced the new appointment. "This matter will receive attention at the national level."

The charges against detective Hilton Botha further embarrassed the prosecution which has seen its evidence repeatedly picked apart during the bail hearing for Pistorius over the Valentine's Day killing of his girlfriend Reeva Steenkamp.

Pistorius himself suffered a new blow on Thursday when US sportswear giant Nike said it has suspended its contract with the double amputee Olympian and Paralympian known as "Blade Runner" who had become an inspiration to millions. The 26-year-old is due back in court on Friday for the fourth day of the bail hearing.

Police revealed Thursday that Botha is facing seven charges for shooting at a minibus taxi in 2011, and he also came under repeated fire from Pistorius's defence team over witness testimony and his handling of the investigation. Defence lawyer Barry Roux said there were "disastrous shortcomings in the state's case".

Prosecutors allege it was a premeditated killing, but Pistorius has said he mistook his 29-year-old model girlfriend for an intruder.

Steenkamp was found by medics in the early hours of last Thursday at Pistorius's luxury Pretoria home covered in bloodied towels and wearing white shorts and a black vest, with bullet wounds to her head, elbow and hip. She died at the scene.

The Olympian and Paralympian sprinter, who has been in police custody for a week, could face months or perhaps years in pre-trial detention if he does not win bail. Pistorius, who said he kept a gun in his bedroom because of fears of violence, denies intentionally killing his girlfriend.

He said in a statement read out in court earlier this week that he had fired at the door of the bathroom where his lover was hiding as he was "filled with horrible fear" that someone had sneaked in through an open window in the dead of night.

Botha was forced to admit this week that Pistorius's claims were "consistent" with the crime scene and on Thursday he said of the investigation: "I'm sure it could have been handled better." He has conceded he did not wear protective clothing when Pistorius's forensic team visited his home, which may have contaminated the scene.

He was further forced to admit that police had not seen a bullet that hit the toilet basin and which was only discovered four days later by the defence forensic team.