LOS ANGELES — Madison Hamburg was 18 years old when he got the call from this sister: Their mother had been stabbed and bludgeoned to death outside her home in the affluent southeastern Connecticut town of Madison.

Now 29, Hamburg has directed a nail-biting and hauntingly personal documentary series about his mother Barbara's murder, which remains unsolved. His directorial debut, "Murder on Middle Beach,"  premieres on HBO Max on Sunday.

Hamburg began filming the series in college as part of a school project. It was just three years after the March 2010 killing, and Hamburg was recovering from a drug addiction and still reeling from his mother's violent end.

"I decided to start navigating this journey into understanding who she was and why the murder might have happened," said Hamburg, who began uncovering a side of his mother's life he never knew about. Each interview he conducted only led to more questions and possibilities. 

When it was time to turn in the project, Hamburg wasn't finished.

"We turned in a trailer — or a teaser — and my professor basically made me promise to never stop working on it and he gave me an A in return," Hamburg said recently from his home in Brooklyn. "And I never stopped working on it."

The genre-busting four-episode documentary is largely told from Hamburg's perspective and explores how his mother's death impacted him and his family. It also delves into potential suspects, including several of Hamburg's own family members. 

The project forced Hamburg to ask the toughest question he could possibly ask a loved one: "Did you kill Mom?"

That question was something he posed to several close relatives after he had been working on the documentary for years and realized they needed to be given a chance to express their innocence.

Hamburg recounted how his crew pressed him to ask, saying, 'If we're going to do this again, this might be your only opportunity to ask that question. And you have to do it for their sake and your sake, bluntly. And you have to do it to elicit an honest, truthful answer out of them because this may be your only chance to exonerate people and … resolve some of the lingering distrust,'" Hamburg said. "Because, you know, I love my family and it is unconditional, but it's really hard when a loved one is murdered and it's unresolved and it's done in a way that leaves a small percentage chance that it could have been done by a loved one."

Part of Hamburg's journey in the documentary includes secretly recording his father and his interactions with investigators who have yet to solve the killing. 

Hamburg said all his family members except his father have watched "Murder on Middle Beach" before its release so they're prepared for any potential fallout or social media reaction. 

In the end, he hopes all the hard work and potential family strife will simply help answer the biggest question of his life: Who killed his mother and why?

"If this series doesn't solve the case, it becomes a tool to get us closer," Hamburg said. "Anybody out there that might remember something, it might jog their memory when they see this, you know, please. There's anonymous ways to reach out. Please reach out because we will greatly appreciate, even if it's just to share how much you loved my mom."