Police initially investigated this claim, but grew suspicious due to the brothers' spending and their hiring of a computer expert to delete their father's recently updated will.

Erik confessed to the murders in sessions with his psychologist, citing a desire to be free of a controlling father with high standards, which led to their arrests months later.

During their first trial, the defense argued that the brothers killed their parents in self-defense after years of alleged sexual, emotional, and physical abuse.

The prosecution argued that the murders were premeditated, that allegations of sexual abuse were fabricated, and that the brothers were motivated by hatred and a desire to receive their father's multimillion-dollar estate after being disinherited from his will.

[8] The highly publicized trials received international media attention, inspiring numerous documentaries, dramatizations, books, and parodies.

[12][9] In 1988, Lyle and Erik committed multiple burglaries in their neighborhood, stealing more than $100,000 in cash and jewellery, prompting José to move to Beverly Hills.

[14] The following year, Erik attended Beverly Hills High School where he earned average grades but displayed a talent for tennis, ranking 44th in the US as a junior.

[19] In high school, Erik co-wrote an amateur screenplay titled Friends with his classmate, telling the story of a wealthy young man who committed the "perfect murder" by killing his parents for their inheritance.

[22][23] However, due to issues with Lyle's California driver's license and a two-week waiting period mandated by gun laws, the brothers decided to purchase shotguns instead.

[24] They acquired Mossberg 12-gauge shotguns along with boxes of birdshot and buckshot ammunition in a Big 5 Sporting Goods store in San Diego, California, where Erik used a stolen driver's license from Lyle's friend, Donovan Goodreau.

[37] Police officers and forensic staff who worked on the crime scene described it as "the most brutal" one they had ever encountered, noting the blood and brain matter splattered throughout the room.

[39] Detectives initially investigated Lyle's suggestion that the murders were a result of mob-related activity due to its heinousness and José's business connections.

[41][14] Lyle bought Chuck's Spring Street Café, a Buffalo wing restaurant in Princeton, New Jersey, as well as three Rolex watches and a Porsche Carrera sports car.

The brothers eventually left the Beverly Hills mansion unoccupied, choosing to live in adjoining condominiums in nearby Marina del Rey.

[50] Lyle was arrested on March 8, 1990, outside their Beverly Hills home, while Erik turned himself in three days later after returning to Los Angeles from Israel.

Meanwhile, their mother was described as an enabling, selfish, mentally unstable alcoholic[58] and drug addict[failed verification] who encouraged her husband's behavior and was also violent toward the brothers.

[3] Prosecution argued the murders were inconsistent with a self defense killing; after they had shot their mother Kitty who was "moaning and trying to crawl away", Lyle went to reload his shotgun and returned to finish her off.

[69] Evidence from a taped therapy session between the brothers and their psychologist, Jerome Oziel, was also presented in court, after legal attempts by the defense to exclude it.

[3] According to Oziel's account, "They didn't kill their parents for money but rather out of hatred and out of a desire to be free from their father's domination, messages of inadequacy, and impossible standards.

[50] Smyth's testimony was challenged by the prosecution, noting she was the one who notified the authorities of the confession tapes, but had now joined the defense to discredit Oziel after a tumultuous breakup.

[72] Prosecutor Lester Kuriyama proposed that José had not forced Erik into sex acts, but was furious that his son was potentially homosexual, and this may have caused tensions within the family.

[75] The prosecution wanted to allow the jury to see a screenplay that Erik had written with his friend Craig in high school, a story about a wealthy young man who killed his parents for the inheritance money.

She testified that during a regular visit to see Lyle in prison, he offered her a large sum of money to claim that José Menendez made sexual advances towards her.

[82] Prosecutor David Conn told jurors that Lyle had asked his friend Brian Eslaminia to fabricate a story in the first trial.

[82] Psychology professor John Wilson, an expert witness for the defense testified that Erik had symptoms of PTSD which supported his allegations of abuse.

[82] Leslie Abramson argued that the brothers acted out fear that their parents would harm them for threatening to reveal the family's secrets,[82] and that the killings were a "highly emotional overkill".

[88] Both brothers also filed motions for a mistrial, claiming that they suffered irreversible damage in the penalty phase as a result of possible misconduct and ineffective representation by Abramson.

[7] In May 2023, the brothers requested a new hearing based upon an allegation that their father had raped former Menudo member Roy Rosselló, who, at that time, was signed under RCA Records.

[92] On October 3, 2024, Los Angeles District Attorney George Gascón announced in a press conference that his office was actively reviewing the appeal.

In November 2003, Lyle married Rebecca Sneed at a ceremony in a visiting area of Mule Creek State Prison; they had known each other for around 10 years before their engagement.