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"You fired four shots through the door whilst knowing that she was standing behind the door," said prosecutor Gerrie Nel, known in South Africa as "The Pitbull" for his hectoring style of questioning.
"She was locked into the bathroom and you armed yourself with the sole purpose of shooting and killing her."
"That is not true," said 27-year-old Pistorius, who faces life in prison if convicted of murder.
Pistorius has broken down in tears on many occasions during the questioning, and at one point retched into a bucket on the witness stand after being shown grisly pictures of Reeva Steenkamp after the shooting on Valentine's Day last year.
He insists he killed the 29-year-old law graduate and model accidentally after mistaking her for an intruder hiding behind a closed toilet door.
He told the court he had pulled the trigger without thinking after hearing a noise behind the door, out of terror and fear that his and Steenkamp's lives were in danger.
"I was extremely fearful, overcome with a sense of terror and vulnerability," said Pistorius, whose lower legs were amputated as a baby.
"I didn't think about pulling the trigger, as soon as I heard the noise, before I could think about it, I pulled the trigger."
The athlete's voice quivered as he recounted how he was "overcome with terror and despair" on finding her bloodied body slumped against the toilet after he broke down the door with a cricket bat.
"I was broken, I was overcome, filled with sadness," he told judge Thokozile Masipa, adding he urged Steenkamp to hold on while he sought help from neighbours at his high security Pretoria residence.
Pistorius insists he and Steenkamp were in a loving, if fledgling, relationship, despite phone text messages read in court which pointed to some arguments. On Tuesday he read a Valentine's Day card his girlfriend got for him before her death.
"Roses are red, violets are blue," the card begins.
"I think today is a good day to tell you that I love you," the message concludes, the last part in Steenkamp's own words.
The trial has drawn wide interest both in South Africa and abroad.
Before the shooting, Pistorius was one of South Africa's most revered sportsmen, admired for his prowess on the track using carbon-fibre prosthetics that earned him the nickname "The Blade Runner" and brought him a clutch of Paralympic medals.
The defence later moved onto questioning its third witness, with the trial looking likely to run into next month.