Jennifer and Sarah Hart, with their six adopted children. (Facebook)

A new documentary will investigate the shocking murder-suicide of a lesbian couple and their six children, after one mother intentionally drove their car off a cliff.

A Thread of Deceit: The Hart Family Tragedy, released on streaming platforms on Tuesday (April 7), will explore the case of Jennifer Hart, who in 2018 drove her wife Sarah and six adopted children off a cliff on the Pacific Coast Highway, California, and into the ocean.

Five of the children – Markis, 19, Hannah, 16, Jeremiah, 14, Abigail, 14, and Sierra, 12 – were found dead in or nearby the SUV. The body of their sixth child, Devonte, 15, was never found, but Mendocino County sheriff-coroner Tom Allman later ruled that he was in the vehicle at the time of the crash, allowing his death certificate to be signed.

Although the crash was initially considered an accident, it was later ruled a murder-suicide as more evidence surfaced.

According to the BBC, Jennifer and Sarah researched suicide online before the crash, which came days after a child abuse inquiry was opened against them.

Sarah had already pleaded guilty to abusing one of her daughters in 2011 after she hit her over a tub, leaving bruising on her stomach and back. She was charged with domestic assault and sentenced to a year of community service.

Toxicology reports after their deaths showed that Jennifer was driving over the legal alcohol limit, and that Sarah and two of the children had diphenhydramine in their systems, an antihistamine that causes drowsiness.

California highway patrol investigator Jake Slates said in the coroner’s report: “They both decided that this was going to be the end. That if they can’t have their kids that nobody was going to have those kids.”

A Thread of Deceit will take un ‘unbiased’ look at the tragedy.

The new documentary, from producers Chris Kobin and Rachel Morgan, will explore the events surrounding the murder-suicide, “peeling away the layers of something that was unfathomable”.

According to Advocate, Morgan said: “When I heard of the Hart tragedy in March of 2018 I was living in Los Angeles, but I had spent about a decade up in Portland, Oregon and knew a handful of people that were close to the family.

“The thing that struck me the most, aside from it being an absolutely horrific, heartbreaking incident, is how quickly the story was scooped up and spit out in the mainstream and social media with very little compassion and human decency. This is what motivated the documentary.

“It was important for us to go into the documentary unbiased and let the story tell itself in any way that it took form.

“I felt it crucial to allow the friends who were silenced by harassment to be able to speak their point of view and I felt it was just as important to share the cold hard facts from the investigation and inquest without sugar-coating it.”