Murphy, whose biggest films included Clueless, 8 Mile and Just Married, died aged 32 in 2009.
Her cause of death was ruled an accident, with a coroner blaming a combination of pneumonia and over-the-counter medications she had taken.
Two-part documentary series What Happened, Brittany Murphy? features interviews with Murphy’s former co-stars and shines a light on her marriage with London-born screenwriter Monjack.
A trailer accuses Monjack of repeatedly lying about having survived terminal brain cancer and having dated Madonna and Elle MacPherson.
To add to the mystery, he died aged 40 less than six months after Murphy at the same Los Angeles house under similar circumstances.
“Brittany was so wonderful,” actor Kathy Najimy, who worked on animated series King of the Hill with the late star, said in the trailer.
She said Monjack “came along” when the star was suffering a “little dip” in her personal life.
Najimy added: “I think that’s where her judgment was muddled, and she became prey. We all were scared and freaked out. Like, ‘Who was this guy?’”
Elsewhere in the trailer, doubt appears to be cast on the narrative surrounding Murphy’s death.
“A healthy 32-year-old doesn’t just die,” a contributor says.
Directed by Cynthia Hill, What Happened, Brittany Murphy? features new archival footage and interviews with those closest to the actor.
Producers say it “goes beyond the tabloid rumours to examine the impact of Hollywood’s sexism on her struggles, as well as lingering concerns about her relationship with” Monjack, who married Murphy in 2007.
Both episodes of the documentary will debut on HBO Max on 14 October. A UK release date has not been announced.
Additional reporting by Press Association