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Defence lawyer Barry Roux then read a sworn statement from Mr Pistorius, saying the couple had decided to spend that evening at home.
They were in the bedroom when at one point Oscar Pistorius went to the balcony and heard a noise, the statement said.
"It was pitch-dark in the bedroom," it added. "I did not have my prosthetic legs on and felt extremely vulnerable."
According to the statement, the defendant thought Ms Steenkamp was still in bed, and he fired through the bathroom door believing he was shooting at an intruder.
When he realised Ms Steenkamp was not in the bed, "it filled me with horror and fear", it said.
Mr Pistorius broke the door down with a cricket bat to find his girlfriend shot inside, the statement said.
Oscar Pistorius could face life in jail if found guilty
The athlete was sobbing in court, correspondents say.
The hearing was later adjourned until Wednesday.