June Steenkamp says her daughter Reeva never had sexual relations with Oscar Pistorius and was considering leaving him when he shot her on Valentine's Day 2013.
Speaking to The Times magazine this weekend, June described the 29-year-old Paralympian as "pathetic", "moody", "gun-toting" and "possessive" and claimed he "would have killed someone sooner or later".
Reeva, a 29-year-old model and law graduate, died in a toilet cubicle in Pistorius' house after the sprint runner fired four shots through the door just after 3am on Valentine's Day last year. The South African athlete was sentenced to five years in prison for culpable homicide last week but is expected to serve just 10 months of that time behind bars.
The Times is serialising June's book, Reeva: A Mother's Story, out on the 6 November.
The 68-year-old was a constant presence during Oscar's trial for murder in Pretoria High Court, although she did not testify against him.
She told the newspaper that Reeva had expressed "nagging doubts" about her and Oscar's compatibility.
"She had confided to me that she hadn't slept with him. They'd shared a bed, but she was scared to take the relationship to that level," June said. "She wouldn't want to sleep with Oscar if she wasn't sure. I believe their relationship was coming to an end. In her heart of hearts, she didn't think it was making either of them happy."
She added that Reeva was about to leave Oscar on the night of her death: "Her clothes were packed. There is no doubt in our minds: she had decided to leave Oscar that night."
Oscar Pistorius was previously acquitted of murder after Judge Thokozile Masipa accepted his claim that he had shot Reeva "without thinking", believing her to be an intruder.
But June and Reeva's father Barry remain convinced that he is not telling the truth and that something else happened to trigger their daughter's "horrible, painful, terrible" death that night.
"There is no doubt in our minds that something went horribly wrong, something upset her so terribly that she hid behind a locked door with two mobile phones," June writes in her book.
Also in the book, June describes how she was unimpressed by Oscar's apology to her and Barry during his trial.
"Why decide to say sorry to me in a televised trial in front of the whole world? I was unmoved by his apology.
"I felt if I appeared to be sorry for him at this stage of his trial on the charge of premeditated murder, it would in the eyes of others lessen the awfulness of what he had done. He was in the box trying to save his own skin."
June previously said that there was still "a big missing piece of the puzzle" regarding their daughter's violent death and that only Oscar knew the truth of what really happened to her.
"We're not completely happy with the evidence that came through," she told ITV last week. "There's still a big missing piece of the puzzle. We feel there's more to the whole story than was stated."
But she said she was "satisfied" with the sentence passed down and was not seeking vengeance.
"He has got to pay for what he has done. It is not that we want vengeance or anything, or him to suffer with his disabilities, but at the same time we feel satisfied that he will realise now that you can't go around doing things like that.
"He will suffer but [it will be] more mental pain than anything... now he has to pay for what he's done to our daughter."
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