Police said Steenkamp was shot four times in the predawn hours in the home, located in a gated community in Pretoria, reports SI.com, which described Steenkamp as a model who spoke out on Twitter against rape and abuse of women.
Steenkamp's publicist Sarit Tomlinson said, "Our thoughts and prayers go to the Steenkamp family, who have asked to have their privacy respected during this difficult time, everyone is simply devastated. She was the kindest, sweetest human being; an angel on earth and will be sorely missed."
CNN reports that a pistol was recovered at the scene, according to police. Early news reports, unconfirmed by police, said Steenkamp was planning a Valentine's surprise for Pistorius, 26, but that it went horribly wrong.
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"Yes, there are witnesses, and there have also been interviews this morning," Beukes told reporters outside Pistorius's gated community. "We are talking about neighbors and people that heard things that happened earlier in the evening and when the shooting took place."
The suspect (who initially was not named, per South African law) was being cooperative and undergoing blood alcohol and forensic tests, said Beukes, adding that he requested to be brought to court immediately and that an application for bail would be refused.
Pistorius's court hearing was originally scheduled for Thursday afternoon but has been postponed until Friday to give forensics investigators time to carry out their work, said Medupe Simasiku, a spokesman for the prosecution.
A 9 mm pistol was recovered and a murder case opened against Pistorius. Hours later after undergoing police questioning, Pistorius left a police station accompanied by officers. He looked down as photographers snapped pictures, the hood on his gray workout jacket pulled up, covering most of his face.
A spokeswoman for Pistorius declined to comment, but the athlete's father, Henke, told the South African Broadcasting Corporation that his son was "sad at the moment."
Henke Pistorius added, "It will be extremely obnoxious and rude to speculate. I don't know the facts."
Chris Collingridge / AP