PRETORIA, South Africa—South African police brought in their most-senior detective to lead the murder case against Olympic athlete Oscar Pistorius, because the previous lead investigator himself is facing attempted-murder charges.

The withdrawal of chief investigator Hilton Botha, who was under pressure from defense lawyers for his handling of the investigation, is a sharp twist in the state's case against Mr. Pistorius over the alleged premeditated murder of his 29-year-old girlfriend, Reeva Steenkamp, a model and reality...