Lyle and Erik Menendez leave a courtroom in Santa Monica in 1990. The brothers were found guilty of first degree murder and conspiracy of the murders of their parents.

On the outside, the Menendez family appeared to be living the American dream, but on the inside, they were all living a nightmare.

Now, for the first time in 20 years, Lyle Menendez — who, along with his brother, is serving life in prison for killing his parents — has spoken out about the brutal attack that shook America.

Menendez and his brother Erik, who at the time were 21 and 18 respectively, shot their sleeping parents Jose and Mary execution-style with 12-gauge shotguns in the den of the family’s Spanish-style, Beverly Hills mansion on the night of August 20, 1989.

Mary ‘Kitty’ Menendez had her face blown off, while her husband suffered “explosive decapitation with evisceration of the brain”, the autopsy revealed.

The sons originally told police they discovered the bodies when they arrived home several hours later. But in court, both brothers admitted while on the stand to walking into the family room, shotguns loaded, and firing at their wealthy parents seated on the couch in front of them.

“Something happened during the course of their childhood that turned them into murderers,” family friend Alicia Hertz told ABC. “These two terrorist parents built two bombs that blew up and killed them,” a jury member who was present during the trial added.

The horrific killings of the wealthy Cuban-American business executive and his socialite wife became one of the most sensational murder trials of the 1990s. But it took seven years and two trials before the truth was finally revealed and the Menendez brothers were jailed for their crime.

In a two-hour documentary special that aired in the US on Thursday night, the now 48-year-old Lyle opened up to ABC News about his father, speaking with surprising passion about the man who raised him — a man who he accused of sexual assault.

“My father was like a force of nature and you look up to that,” he told ABC from Mule Creek State Prison in California where he will likely spend the rest of his life. “You are expected to be competitive to a fault. He began to sort of try to groom me to be hypercompetitive and succeed.”

Jose Menendez, 45, a Cuban immigrant who had worked his way up to become a powerbroker in Hollywood, had amassed a fortune. “Jose was very driven to become the American success story,” a neighbour recalled to ABC.

Not everything was as it seemed.Source:Supplied

“He came to America as a teenager, with this ferocious drive and talent.”

Another said, “Everyone described Jose Menendez as someone you should be afraid of.”

At the time, the brothers’ defence team alleged that the siblings acted in self-defence after suffering years of sexual abuse from their father. However prosecutors said that the brothers killed their parents in an attempt to obtain the family fortune and the second trial saw the pair convicted. They said the abuse was fabricated, calling it “the abuse excuse”.

Journalist Robert Rand, who covered both the original investigation and the trials of the Menendez brothers, painted a picture of two spoilt rich kids, who grew up in a mansion in Calabasas and were offered the world, but chose to throw it away.

“They were in a culture where money can buy you anything … it can buy you a free pass out of trouble,” he said. “I think the Menendez brothers were close because they were fighting a common enemy and that was their father.”

He added: “I think Jose thought that life was about winning and probably it was not important how you got there … These were his prize thoroughbreds, his sons, they were going to reflect his glory and if they didn’t, God help them.”

“They were seen as rich kids, a step above everybody else. Princeton was about old money, you didn’t show off. But they were different,” he added.

“They were very concerned about the facade of their family, they wanted the public image to be perfect and one of the ways they did this was to do their kids’ homework. Another way was to choose who they could date.”

It was also revealed that Jose Menendez was having an eight-year affair with a woman in New York, as well as an affair with another woman in Los Angeles. The knowledge of these affairs led Kitty to overdose on valium in an attempted suicide, friends said.

Terry Moran, the ABC journalist who covered the story nearly three decades ago, said: “They said they did it because they’d been sexually abused, so the question in the trial — if you believed they had been sexually abused — was, ‘does that lessen their sentence?’”

In the months after their parents’ murders, the brothers began to spend their inheritance. They bought expensive cars, Rolex watches, high-end condos — even a restaurant in Princeton, where they went to school. The family estate was valued at $14.5 million at the time, it is believed the bothers blew at least $700,000 of their fortune.

The murder scene where Jose and Kitty Menendez were brutally gunned down in their Beverley Hills home. Picture: Screengrab/abc newsSource:Supplied

Their case quickly became the original ‘trial of the century’ before O.J. Simpson’s trial and the first — which ended in mistrial — was highly publicised, being broadcast nationwide on the newly-founded Court TV in 1993. At the second trial in 1995, TV cameras were not permitted inside the courtroom.

“It’s shocking to think ... that I could have been involved in taking anyone’s life,” Menendez told ABC over the phone. “I found that my own childhood prepared me surprisingly well for the chaos of prison life.”

“I’m a more fully formed adult now. Of course, looking back it’s shocking to think about what happened. That I could have been involved in taking anyone’s life and my parents’ life,” he added. “It’s still jarring … it seems unimaginable because it seems so far removed from who I am and who I was.”

The brothers were sentenced to life behind bars in March 1996 and are serving their time without parole in two separate prisons. They haven’t seen each other since a 1996 interview with Barbara Walters, although Lyle said the pair write letters to each other and that their “bond is really strong”.

“I am the kid that did kill his parents, and no river of tears has changed that and no amount of regret has changed it,” he said. “I accept that. You are often defined by a few moments of your life, but that’s not who you are in your life, you know. Your life is your totality of it … You can’t change it. You just, you’re stuck with the decisions you made.”

The gripping documentary, titled Truth and Lies: The Menendez Brothers — American Sons, American Murderers, interviewed several people who were close to the family when the brutal crime took place.

Among them was Diane Vander Molen, the cousin of Lyle and Erik Menendez, who spoke out for the first time since she testified at their trial in 1993. Ms Vander Molen said she has no doubt the brothers were sexually abused by their parents.

“I know that they would never, ever have done what they did unless they felt that they had no choice, that it was either them or their parents,” she told ABC News. “I believe that very strongly.”

Erik, left, and Lyle Menendez leave the memorial service for their slain parents in August, 1989.(AP Photo/Los Angeles Times, Steve Dykes)Source:News Limited

Ms Vander Molen, who holidayed with the family, testified at the brothers’ first trial that Lyle confided in her about the alleged sexual abuse by his father in the summer of 1976. At the time, Lyle was eight years old and his cousin was 17.

“One night, I was in my room changing the sheets in my bed, and Lyle came in,” she said. “He became very serious about asking me if he could sleep in the other bed next to mine and saying that he was afraid to sleep in his own bed because his father and him had been touching each other down there, indicating that it was his genital area.”

Ms Vander Molen said she later told their mother Kitty what her eldest son had confessed. “By her demeanour, I could tell that she was not believing any of this,” she said. “And [she] went downstairs, and Lyle had already gotten into the bed next to mine, and she went ahead and yanked him by the arm and took him back upstairs and I never heard anything else about that.”

She added: “He was scared to sleep in his own bed because he was afraid that his father was going to come in and molest him that night.”

She said Lyle never spoke to her about it again.

“I know for 100 per cent that there was [sexual abuse],” Ms Vander Molen said when asked why she decided to defend her cousins. “Their privacy was everything to them. They were completely different people when nobody was around. And then Jose and Kitty would turn on the charm when they had people over, which wasn’t very often.”

Kitty’s brother, Brian Anderson, also spoke to ABC about the murder of his sister at the hands of her two sons. He strongly denied his nephews’ claims that they were abused at home.

“There was certainly no indication of any kind that there was ever any abuse, Mr Anderson told ABC. “It just didn’t happen, it just didn’t happen. I think the motive was strictly money.”

He added, “Jose and kitty were both having second thoughts about being so generous … Kitty essentially realised that they had raised a playboy [in Lyle], so what they did was take his credit cards away. Lyle was very upset.”

Karen Farrell, a close friend of Kitty’s recalled seeing her the week before she was murdered. “She seemed happy, she said she was getting along with Jose. She thought everything was doing better,” Ms Farrell said.

In the decade following their conviction, the Menendez brothers appealed their case up to the US 9th Circuit Court of Appeals but were denied each time.