It was a riveting case that attracted a daily national audience to a televised criminal trial. Three decades ago, two wealthy young men were charged with murdering their parents by marching into their Beverly Hills mansion’s den with shotguns and unloading more than a dozen rounds on their mother and father while they sat on the couch.

Lyle and Erik Menendez were convicted in 1996 of murdering their mother, Mary Louise, a former beauty queen known as Kitty, and father, Jose, a music executive, despite defense claims that the brothers had been sexually abused by their father for years and killed out of fear.

Roy Rosselló, a former member of Menudo, the 1980s boy band that became a global sensation, is now coming forward with allegations that Jose Menendez sexually abused him as a teenager.

The claim was made on Tuesday during a segment on the “Today” show that detailed some of the findings of a three-part docuseries set to premiere on Peacock, NBCUniversal’s streaming service, on May 2.

“Menendez + Menudo: Boys Betrayed,” based on reporting by journalists Robert Rand and Nery Ynclan, focuses primarily on Mr. Rosselló. He recounts an encounter with Mr. Menendez, but also separate incidents of sexual abuse inflicted on him by one of Menudo’s former managers when he sang with the group.

Mr. Rosselló says of Mr. Menendez, “I know what he did to me in his house,” in a clip from the docuseries that aired on “Today.”

It’s unclear what effect, if any, Mr. Rosselló’s account will have on defense lawyers’ efforts to secure a new trial for the brothers, whose previous appeals were denied.

The credibility of the brothers’ account, as well as the admissibility of defense arguments pointing to sex abuse as a mitigating factor in the case, were central to the criminal trials that took place after the murders were discovered in 1989. The first trial, which began in 1993, resulted in two hung juries and two mistrials. When the brothers were retried two years later, they were found guilty of first-degree murder and sentenced to life in prison, where they are still serving their sentences.

The District Attorney’s Office of Los Angeles County, which prosecuted the cases in the 1990s, did not immediately respond to a request for comment early Tuesday morning.

According to the “Today” report, Mr. Rosselló is expected to describe a visit to the Menendez home in New Jersey when he was 14 years old, during which he claims Jose Menendez drugged and raped him.

“That’s the man here that raped me,” he says in a clip of the docuseries, pointing to Mr. Menendez in a photo. “That’s the pedophile.”

He is also heard saying, “It’s time for the world to know the truth.”

Mr. Menendez was associated with Menudo because he had signed the group as an RCA Records executive.

Mr. Rosselló has previously stated that he was sexually abused as a member of Menudo. Others have also claimed to have been verbally, physically, emotionally, and sexually abused while performing in the four-part HBO Max docuseries “Menudo: Forever Young.” No one has ever been charged criminally in connection with the allegations.

Milton Andersen, 88, one of Kitty Menendez’s brothers, used an expletive to describe Mr. Rosselló’s allegation as flatly false and said the Menendez brothers should not be released.

Mr. Andersen claimed his brother-in-law was not a sexual predator and argued that the new accusation could lead to Lyle and Erik’s case being re-examined.

“They do not deserve to walk the earth’s surface after murdering my sister and brother-in-law,” he said.

The Menendez murders drew widespread public attention, in part because the brothers were wealthy children. Lyle was a student at Princeton at the time of the murders. Erik was aspiring to be a professional tennis player. Prosecutors portrayed them as cold-blooded killers seeking unrestricted access to their parents’ $14 million estate.

Jose Menendez was shot five times in the head, including once in the back. According to the brothers’ testimony, after firing several rounds, Lyle went to his car, reloaded his 12-gauge shotgun, and pushed the muzzle of his gun against his mother’s cheek, shooting her again.

The police initially suspected the slayings were connected to the Mafia. However, investigators focused on Lyle, who was 22 at the time of his arrest, and Erik, 19, after the brothers purchased Rolex watches, condominiums, sports cars, and other items in the months following the murders.

Though they initially denied any involvement in the killings, they became the primary suspects following the discovery of taped recordings of conversations the brothers had with their psychologist during which the brothers explained what had led them to murder their parents.

As the first trial approached, the brothers’ defense attorneys presented their own explanation for the crimes: Lyle confronted his father about the family’s sex abuse secrets, his father became enraged and threatening, and the brothers killed out of fear for their lives.

The defense argued that the murder charges should be reduced to manslaughter because the defendants honestly, if incorrectly, believed their lives were in danger.

The trials, which aired on Court TV, marked the beginning of a new era of televised courtroom drama. At least some jurors in the first round of trials believed the brothers, who had testified movingly about their abuse. The testimony divided the jurors between manslaughter and murder verdicts, contributing to the impasse that resulted in the mistrials.

The circumstances had changed by the time another jury convened to decide the brothers’ fate. The judge barred cameras from the courtroom and severely limited witness testimony and evidence concerning Jose Menendez’s parenting. Prosecutors, who had let the brothers’ molestation accusations go unchallenged in the first trials, pounced on Erik Menendez when he took the stand, casting doubt on whether the abuse had occurred at all.

“Can you give us the name of one eyewitness to any of the sexual assaults that occurred in that home?” asked the lead prosecutor, David Conn, as he ticked off the locations where the brothers had lived.

According to testimony transcripts, Mr. Menendez kept repeating the same answer: “No.”

In addition, the defense did not present anyone at trial other than the brothers who described Mr. Menendez as a sexual predator.

As the trial came to a close, Judge Stanley M. Weisberg ruled that the “abuse excuse” could not be used. The decision essentially forced jurors to choose between acquitting the brothers or convicting them of murder.

They chose the latter.

“We did believe there was some psychological abuse.” “I believe most of us believed that,” one juror, Lesley Hillings, later told The Los Angeles Times. “Sexual exploitation?” I doubt we’ll ever know whether that’s true or not.”

Legal experts said that even with Mr. Rosselló’s new allegation, the lawyers defending the Menendez brothers would face an uphill battle if they attempted to have the case re-examined.

Mr. Rosselló’s information, according to Laurie L. Levenson, a professor of criminal law at Loyola Law School in Los Angeles who provided legal analysis of the Menendez case in the 1990s, may be “too little, too late.”

“In the end, in the second trial, the jury just didn’t believe them,” Professor Levenson said of the brothers’ allegations of sex abuse.

Mr. Rosselló’s story “could be something you could file with the court and claim that it’s newly discovered evidence and that it would have made a difference in the case,” she said. “However, they will bear the burden of demonstrating that.”

In the segment aired by “Today,” Alan Jackson, a criminal defense lawyer, agreed that the brothers had “a big mountain to climb.” Still, he said the assertion brought forward by Mr. Rosselló provided the brothers a “glimmer of hope.”