Faith Harris married a convicted murderer. Six years later, all her children were dead.

On September 21, 2016, Faith Harris-Green woke up in the basement of her home.

She was found with duct tape and zip ties, had been shot in the foot and her face had been slashed with a box cutter.

She watched as her husband, Gregory Green, murdered her two teenage children, his stepchildren, 19-year-old Chadney Allen Jr. and Kara Allen, 17. Green had also killed the couple’s two young daughters, aged five and four.

This wasn’t the first time Green had killed: in 1991, he stabbed his wife in the face and chest, killing her and their unborn child.

He then called 911 and waited for the police to arrive and arrest him.

He served 16 years in prison for the murder. He was released on parole in 2008, after family, friends and his pastor – Faith Harris-Green’s father – campaigned for his release.

"Gregory and I were friends before his mishap and he was incarcerated," Faith's father, Fred Harris, wrote to the Michigan parole board in August 2005, the Washington Post reports.

"He was a member of our church...I feel he has paid for his unfortunate lack of self control and the damage he has caused as much as possible and is sorry."

A year later, Harris wrote that Green would be "welcomed as a part of our church community".

LISTEN: Charlie Bezzina talks about Melbourne’s underworld and what it takes to be a criminal investigator.

After his release, Green and Faith Harris married, and they had two daughters together.

Just as he had years earlier, Green called the police after he murdered the four children on September 21, 2016.

Police found his youngest daughters in their beds upstairs: he had poisoned them with carbon monoxide.

Veteran law enforcement officials described the scene as one of the worst they had seen in their careers.

Gregory Green pleaded guilty to the four slayings when he appeared in a Detroit court last week. He also pleaded guilty to the attack on his then-wife, Faith.

"Justice will come when you burn in hell for all eternity for murdering four innocent children, Faith Green during her ex-husband's trial.

Faith Green speaks about the deaths of her four children in court. Watch here. pic.twitter.com/8Z9il536cy

— Elisha Anderson (@elishaanderson) March 1, 2017

"You're a con artist. You're a monster. You're a devil in disguise. You are now forever exposed.

"I miss my children so much that words will never be able to explain...There's a hole in my heart and soul that can never be repaired. This wound will never heal."

Gregory Green will spend 47-102 years behind bars for the murders. He won't be eligible for parole until he is 97 years old.