Hilton Botham, who is now retired, found Pistorius' shirt and shorts covered with blood when he interviewed the athlete known as "the Blade Runner," ABC reported.
Botham said Pistorius thought he heard a burglar when he fired into a bathroom door at his home.
Prosecutors charge Steenkamp was shot after the two argued.
"Steenkamp fled the bathroom and Pistorius went and got a gun, her friends told me," Botham said.
The detective said Pistorius, 26, told him he had washed his hands after he attempted to perform life-saving measures on his girlfriend.