"I think I was just absolutely broken down with stress," he tells NBC News' Keith Morrison in an exclusive clip from Friday's Dateline

Lyle Menendez was sobbing hysterically as he spoke with 911 on Aug. 20, 1989, in a call to report the deaths of his parents, José and Kitty, who had been killed by shotgun fire.

“They shot and killed my parents!” he cried to the dispatcher.

But speaking out this year, more than two decades after he and his brother, Erik, were convicted as their parents’ murderers, Lyle says he wasn’t “grief-stricken” on that call at all.

“I think I was just absolutely broken down with stress,” he tells NBC News’ Keith Morrison in an exclusive clip from Friday’s Dateline, which re-examines the Menendez brothers case.

Both of us were just in such a state of trauma and I just — it just poured through on that call,” Lyle says. “It made it very easy to make that call, really.”

The investigation showed that Lyle and Erik — 21 and 18 at the time, respectively — were armed with 12-gauge shotguns when they burst into the den of the family home in Beverly Hills, California, while their parents watched TV.

By the time the gunfire was over, the pair had shot their parents more than a dozen times. They fired on father José at point-blank range and kept shooting at Kitty as she attempted to flee.

Prosecutors said the brutal slayings were part of a larger scheme to get the Menendez parents’ $14 million estate. But the brothers claimed, in detail, that they acted in self-defense after years of abuse.

• Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter.

F:PHOTOMediaFactory ActionsRequests DropBox47166#GettyGettyImages-114281530.jpg
From left: Lyle and Erik Menendez with defense attorney Leslie Abramson in court on Aug. 12, 1991
| Credit: MIKE NELSON/AFP/Getty

Jurors were unconvinced and Erik and Lyle were convicted of first-degree murder in 1996, after which they were sentenced to life in prison without the possibility of parole.

For years, neither brother spoke often from behind bars — though Lyle, now 49, has become increasingly outspoken in recent months and Erik, now 46, is set to share his lengthiest interview yet, in an upcoming A&E documentary.

Friday’s Dateline will include interviews with a former detective and prosecutor on the case, former members of the jury and others. It will also feature a letter from Lyle to Erik.

Speaking to Keith Morrison, Lyle says in the episode that he still maintains their crimes were distorted by authorities.

• PEOPLE’s special edition True Crime Stories: 35 Real Cases That Inspired the Show Law & Order is on sale now.

“There are, like, 200 to 300 hundred parricide cases a year, where a parent is killed by a child. And they are almost all related to abuse. And they are almost all settled. This case they picked out as different.” he says, “And I think that it was very easy, because it was Beverly Hills and my father had a lot of money, to sort of sell this headline that these brothers killed for money.”

RELATED VIDEO: Exclusive New Insight Into Menendez Brothers ‘Murder Made Me Famous’ Story

But Morrison asks Lyle why he wasn’t more forthcoming on the 911 call after killing his parents — why didn’t he mention the abuse claims then?

Lyle laughs lightly before he answers:

“I don’t think I was going to tell the Beverly Hills Police Department that I killed my parents and here’s why and they were going to go, ‘Okay, go back home.’ ”

Dateline‘s “Unthinkable: The Menendez Murders” airs Friday (9 p.m. ET) on NBC.