Immediately following the murders of Jose and Kitty Menéndez, their sons Lyle and Erik were not considered suspects. The brothers were the ones to call 911 upon discovering the bodies and they were visibly grief-stricken, sobbing and hysterical. They were so upset that though the police interviewed them, they didn't even bother to test either man for gunshot residue. However, several months later the brothers were arrested for the murder of their parents. So what evidence was there against the Menéndez brothers?
Though the brothers raised suspicions after the murders by going on a spending spree, but they weren't being officially investigated at that time. They might have even gotten away with the crime had Erik Menéndez not confessed to it. Apparently overwhelmed by what they had done and suffering from nightmares, Erik spilled the story to his psychiatrist L. Jerome Oziel. Oziel shared the news with the woman he was seeing at the time, Judalon Smyth, and even called Lyle in to discuss the matter further. That was when Oziel recorded the sessions, getting the brothers' confessions on tape.
The tapes became a point of contention during the brothers' trial. Smyth was the one who eventually brought the information to the police, but doctor/patient confidentiality meant the tapes wouldn't necessarily be admissible in court. However, there were two reasons why the information gathered on the tapes was eventually deemed admissible.
The first what that Lyle, upon hearing that Erik had told Oziel everything, threatened the psychiatrist's life; that allowed for them to break the confidentiality agreement. Then, because the defense put such an emphasis on the brothers' mental state, the tapes became relevant and were allowed in court. Both Oziel and Smythe (who was alleged to have overhead the sessions, something she later denied) testified against Lyle and Erik with information the brothers themselves had shared.
That was where the brothers' defense began to fall apart. Though they insisted that they had killed their parents due to a lifetime of abuse and fear, there were no mention of that on the tapes. Erik had been having sessions with Oziel for several years but never brought up being abused by his father. Instead, in the session with Oziel, Lyle and Erik attributed the murder to a deep hatred for their father and pity for their mother, feeling that though she didn't deserve to die killing her put her out of her misery.
Though legally questionable, the taped confessions were instrumental in the arrest and conviction of the Menéndez brothers. Without them, they might never have been caught.