The son of Mr Cheng told The Daily Telegraph his “biggest worry” was the family now won’t be available to “explain how their son (Farhad Jabar Mohammed) became radicalised and the motive that drove him to kill my father”.

Three men charged in connection with the killing — including two accused of supplying the gun — won’t stand trial until next year.

“We need that to understand his death,” Canberra schoolteacher Alpha said.

“It’s a real worry they won’t be able to inform the coroner’s inquest or the trial of the men charged for my father’s death.

“Who’s going to tell us now why Farhad killed my dad?”

The Daily Telegraph can reveal the teenage terrorist’s mother Ghabeliat Omarzadeh quit her North Parramatta unit late last year.

She then returned to her native Iraq with her eldest son — 23-year-old Farshad Mohammed.

The devout Muslim felt harassed and ostracised after her radicalised 15-year-old son fired a bullet into the back of the police accountant’s head outside Parramatta police HQ in 2015.

It leaves no family members left in Australia.

Farhad was shot down by police after executing Mr Cheng.

One year later his sister Shadi Mohammed and her husband Abu Saad al-Sudani — described by the government as “active recruiters of foreign fighters on behalf of ISIL” — were killed in a US air strike on the Syrian city of Al Bab.

The Daily Telegraph can also reveal the patriarch of the family — tradie Jabar Mohammed — also quit Australia weeks after his son was shot dead when the Kurdistan Regional Government in Iraq rejected the family’s bid to bury their son in his Islamic homeland.

Alpha said he was “partly sorry” the mother “felt persecuted for what her son did”.

“Our family is still coming to terms with my father’s death,” he said.

“For me it’s still raw. Some days are more painful than others.

“It has been particularly devastating for my mother.

“She will be shocked and uneasy to learn they have gone.

“Australia and other countries need to find out what drives domestic extremism and Farhad’s family hold the key to that.

“With terrorism and extremism, what it does is destroy families on all sides. There is nothing glorious about it.”

Three men charged over Mr Cheng’s killing — 24-year-old, Talal Alameddine, 23-year-old Mustafa Dirani and 21-year-old Milad Atai — are committed to stand trial next year charged with committing an act in preparation for a terrorist incident.

Prosecutor’s will allege Alameddine and Dirani supplied the .38 Smith & Wesson handgun the teen used for the October 2015 murder.

It is alleged the revolver, thought to have cost $700 on the black market, was handed over in the women’s section of Parramatta Mosque after the teen met several men there hours before he launched his rampage.

Sydney’s Muslim community immediately distanced themselves from the family with the Imam of the Parramatta Mosque Neil El-Kadomi telling The Daily Telegraph: “We do not mention that family’s name in this mosque, they are not part of us and never will be.

“What their son did our community, we will never accept.”

Farhad’s unmarked grave is forgotten in the Islamic section of sprawling Rookwood Cemetery.

Where rows of expensive black marble headstones bear the names of the dead in section 8 of the 286-hectare graveyard, his grassed-over tomb is identified only among those who know by plastic blue hydrangeas and roses.

Ms Omarzadeh deliberately chose to forgo a $9000 headstone to mark his resting place fearing his grave would be desecrated in reprisals after he was linked to IS and al-Qaeda at a time Islamophobic attacks are raging globally.

Ms Omarzadeh, a housewife, and her partner, both Sunni Muslims of Iraqi-Kurdish descent, emigrated to Australia as refugees 16 years ago in search of a better life.

Farhad and his older brother and sister Mohammad followed from Iraq as babies and the family became naturalised Australians.

His parents lived a low key existence in a three-bedroom unit on the top floor of a red brick block in North Parramatta’s Buller St and were said to have been stunned at reports their son had been radicalised.

Described as “a lonely boy from a troubled family” their youngest son didn’t socialise with contemporaries at Arthur Phillip High School.

He hadn’t used social media for several years under his own name but previous posts show an ordinary 13-year-old who followed American basketball and Team Ricky Martin on talent show The Voice.

It is believed, however, that he was accessing social media through his Arabic name, Abu Zaid.

The family refused to accept their son’s wrong doing — blaming Australia for “brainwashing” him, friends say.

A family friend, who asked not to be named, said: “Ghabeliat felt badly treated by Australia and believed country ruined her family.

“She hated Australia in the end, she blamed extreme Muslims in Parramatta for Farhad’s death, for turning his head.

“She is in Iraq now and feels guilty for abandoning Farhad’s grave and still prays for his forgiveness.”