An audiotape secretly recorded during a therapy session captured the Menendez brothers revealing the chilling reason behind their motivations for murdering their parents.
The infamous siblings were the subject of one of the highest-profile trials of the 90s after shooting José, 45, and Mary Louise 'Kitty' Menendez, 47, at their lavish Beverly Hills home on 20 August, 1989.
And now, the extraordinary case is being thrust back into the spotlight by Netflix, as it will be the subject of the second series of Ryan Murphy and Ian Brennan's Monster.
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Lyle, now 56, and Erik, now 53, walked into the California property with shotguns before firing six shots at their father, a wealthy entertainment executive, and then delivering a fatal shot to the back of his head.
Their mother, Kitty, was shot ten times in total, leaving police presuming it might have been a mob hit.
This leading line of inquiry worked out well for Lyle and Erik, as they played the grieving sons in front of authorities and had called 911 saying: "Somebody killed my parents!"
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The brothers were sticking to the story that they had been to watch Batman at the cinema before returning home and discovering the grisly crime scene, as they had dumped the guns and bought movie tickets to corroborate this.
This seemed to be enough to throw police off the scent, for a short time - but the siblings soon raised suspicions when they headed on an extravagant spending spree while supposedly grieving their parents brutal deaths.
The pair are believed to have spent a whopping $700,000 by splashing cash on luxury cars, clothes and watches, while Lyle even bought a buffalo wing restaurant in Princeton, and this raised red flags for investigators.
Cops got one of Erik's pals to grill him about the murders while wearing a wire, but he denied it.
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However, during a therapy session with Dr Jerome Oziel, the then-18-year-old ended up confessing to killing José and Kitty on tape - and he spilled the beans to his mistress, Judalon Smyth.
And when the secret lovers later split up, Smyth told the police all about the information she had been entrusted with.
In the recording, which was taped on 11 December, 1989, the Menendez brothers told the therapist that they had killed their mother to put her 'out of her misery', while José deserved to die because his infidelity is what caused her despair, the LA Times reported.
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This bolstered the prosecution's theory that the pair had committed the slayings to get their hands on their huge inheritance, rather than supporting the siblings claim that they had acted in self-defence.
During the murder trial, Lyle and Erik alleged that their father had molested them throughout their childhoods, with each of them sharing extremely graphic descriptions of the horrors which they claimed had taken place.
But prosecutors honed in on the fact that they had made no mention of these allegations while talking to Dr Oziel - and even testimony from two of their cousins which supported their claims didn't convince the jury.
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Despite being the person who alerted police to their crimes, in a surprise turn of events, Smyth also ended up speaking for the defence, claiming that the therapist had 'brainwashed' her into saying she overheard the brothers confession.
Her change of heart didn't help the Menendez brothers case though and ultimately, they were both convicted of killing their parents in 1996 and sentenced to life imprisonment with no possibility of parole.
They were spared the death penalty because they had no criminal record or history of violence.
Some people have renewed hope that the Menendez brothers might be released after Monster drops, due to it bringing the controversial case back into the spotlight.
Monsters: The Lyle and Erik Menendez Story is available to stream on Netflix from today (19 September).
Topics: Crime, Netflix, Parenting, TV and Film, True Crime, US News, Menendez Brothers
Olivia Burke