For twenty years, a killer has roamed free after bludgeoning 69-year-old widow Marea Yann in the head 22 times in 2003 as she sat in the armchair of her Healesville home.

It was a horrific attack, with no signs of a struggle, no forced entry and no defensive wounds. Nor was there any DNA evidence left behind at the large remote countryside estate where Ms Yann lived on her own.

Blood was splattered on opposite walls of the lounge room. It appeared to be a crime of passion.

In 2008, Ms Yann’s son-in-law, Nigerian-born Joseph “James” Chucks Unumadu was acquitted of her murder by a jury.

But the jury did not hear all of the evidence. And Mr Unumadu to this day denies any wrongdoing.

A fresh investigation by Sky News, called Who Murdered Marea?, hosted by reporter Denham Hitchcock, will air for the first time on Wednesday night, revealing evidence and witness testimony that was never heard by the original jury.

Mr Unumadu, who was Ms Yann’s son-in-law, features in an explosive confrontation when approached by the documentary team, which included this journalist.

Ms Yann was a much-loved community-focused “Nonna”, who ran the local op-shop in town, would cook food for the homeless and less fortunate on Christmas, and was constantly giving money and her time to those less fortunate.

Some believe it was that generosity that led her to her demise. Near the time of her death, Marea’s daughter Pauline and her husband, James Unumadu, owed Marea $40,000.

Ms Yann was also the glue that kept Pauline and James together in their “volatile” and “turbulent” marriage - up until three weeks before her death when she filed an affidavit in the Family Court detailing incidents where James Unumadu was “extremely aggressive and hostile”, “manipulative and dishonest” and in which she alleged she had been assaulted by him.

The documentary will air extraordinary allegations about threatening comments on Marea’s life allegedly made by Mr Unumadu after he found out about the affidavit.

In the time Mr Unumadu was married to Pauline, he had three children to three other women.

In a statement to police following the death of her mother, Pauline said Mr Unumadu had threatened a few months prior to their separation he would “kill her” if she left him. This evidence was not heard by the jury.

Pauline has never been accused of any involvement in her mother’s death. She is also of the belief her former husband had no involvement in her mother’s death.

The documentary uncovers new witnesses who never had the opportunity to tell the jury the events that they still vividly remember.

Janet Bridgart never told police about how Ms Yann fearfully recalled to her about times Mr Unumadu allegedly would appear late at night, peering at her through a window.

Ms Bridgart told the documentary Ms Yann confided in her that Mr Unumadu had threatened her with a voodoo doll.

“She was spiritual and she had a faith that went beyond Christianity, it involved spiritualism and she believed in spirits and she believed in the continuity of life after death.

“So to have a voodoo doll from a culture where people really believed in the power of the voodoo doll and sticking pins in it, really scared her,” Ms Bridgart told the documentary.

Relatives also said Mr Unumadu once ordered Ms Yann to speak with a Nigerian witch doctor named “Ba Ba” over the phone due to her role in the breakdown of his relationship with Pauline. The person on the phone allegedly said that something would happen to her shortly after.

And then she was murdered.

These are just some of the many twists and turns explored in the documentary, with a comprehensive analysis of phone data from the time of the murder also examined, as well as extraordinary recollections from friends and family about the fears Marea had for her life prior to her death.

On Tuesday, Victoria Police announced a $1 million reward would be offered to anyone who came forward with new information leading to a conviction.

They hope the reward will prompt someone who has information and hasn’t yet come forward, to do so.

Victoria Police Superintendent Tim Day, a detective involved in the murder investigation following Ms Yann’s death, remembers the crime scene “vividly”.

“It is, without doubt, one of the most horrendous investigations I have seen,” the former homicide squad detective told Sky News.

And he still believes it can be solved. “Somebody out there knows something, somebody out there hasn’t told us the whole truth,” he said.

“Who Murdered Marea?” premieres Wednesday, May 17 at 8pm AEST on Sky News Australia. Watch on Foxtel or stream on Flash.