Details in a 47-year-old murder case are Marc Leclercbeing uncovered. 

The audio diaries of Margo Compton—who was killed execution-style in Oregon in 1977, alongside her twin daughters, Sylvia and Sandra, 6, and her then-boyfriend's son Gary Seslar—will be broadcast for the first time in over four decades in A&E's chilling new docuseries, The Secrets of the Hells Angels. 

"This is really hard," Compton says in a diary clip shared in E! News exclusive sneak peek from the eight-part series. "There's so much I want to tell, about a girl who had gotten married very young and wanted out of that marriage for some very good reasons."

Compton—who was 24 at the time of her death—later reflects on her tumultuous marriage to Hells Angels wannabe Doug Compton, a key figure in Detective Sergeant Mark Graham's 18-year journey to uncovering who was responsible for the killings. 

"That night, he beat me so bad," she recalls in another clip of her audio diary, "my back is still screwed up from it."

Compton, her 6-year-old twin daughters and Seslar, 19, were all found dead from gunshot wounds to their heads in Compton's Gaston, Oregon, home on Aug. 7, 1977. Seventeen years later, Hells Angels leader Robert G. McClure was found guilty of the killings and sentenced to four consecutive life terms in prison. 

In Secrets of Hells Angels, Detective Graham revisits his decades-long journey to bring justice for Compton and her family—a quest he didn't take lightly, according to his informant Michael Thompson. 

"‘Look, I don't really care about you, but I have a passion and my passion is to come to terms with the execution of these two little girls and Margo Compton," Thompson recalled Graham saying to him in their first meeting in an April 23 sneak peek of the series. "And I said, ‘Well, we share that.'"

Catch the new episode of Secrets of the Hells Angels April 28 on A&E at 7 p.m. PT/ 10 p.m. ET. 

For the latest breaking news updates, click here to download the E! News App