In the early hours of 14 February 2013, he said he had got up from bed to bring in some fans he had left outside, in order to cool his bedroom. When he heard a window opening in the bathroom, he said, he thought there was a burglar trying to get in.
Wanting to protect Ms Steenkamp, who was not sleeping, he had gone to get his gun in the dark.
"I whispered to Reeva to get down and phone the police," he said.
He said he then went into the passage without his prosthetic legs and, overcome with fear, started screaming, and shouted for Ms Steenkamp to get to the floor.
'I got to the entrance of the bathroom at the end of the passage where I stopped screaming. At this point I was certain that the intruders were in the bathroom.
I had my pistol in my right hand and peered into the bathroom. I didn't have much mobility on the tiles. I had my pistol raised to the corner of the entrance of the bathroom.
'I noticed that the bathroom window was open. I was with my back against the wall. I wasn't sure if the intruders were in the toilet or around the corner at that point. Then I heard a noise from inside the toilet that I perceived to be someone coming out of the toilet. Before I knew it I had fired four shots at the door.
'My ears were still ringing - I couldn't hear anything - so I kept shouting for Reeva to phone the police.I was still scared to retreat because I wasn't sure if there was somebody on the ladder or someone in the toilet.'I didn't want to believe it could be Reeva inside the toilet. I didn't know what to do. I kicked the door. I was crying out, I was screaming. I have never screamed like that.'
When she did not respond Pistorius said he realised he may have made a terrible mistake and rushed back to the bathroom to break down the door.
After hitting the door, he saw the key inside, unlocked the door and 'flung it open'.There, he found her fatally injured.
'I said "Oh, Reeva". I sat over her and cried,' the athlete testified sobbing loudly and putting his head in his hands and before letting out the barely inaudible cry: 'she wasn't breathing'.