NSW Police civilian employee Curtis Cheng was gunned down in October 2015 while leaving work by 15-year-old Farhad Khalil Mohammad Jabar in an act of religious extremism.
Farhad was then killed by a special constable after he fired more shots at the police headquarters.
Despite his association with persons of interest to terror investigators in the months leading up to the shooting, there was no evidence Farhad held violent extremist beliefs before the attack, Deputy State Coroner Derek Lee found on Friday.
Police found material on the Year 10 student's phone linked to Islamic State but Farhad had only accessed much of the material in the month leading up to the shooting, Judge Lee said.
He noted evidence from teachers and classmates, who said Farhad was a very religious and hard-working boy at school but did not attempt to impose his beliefs on others.
"He regularly attended Parramatta Mosque ... and had considerable knowledge of the Koran," the coroner said.
"He did not involve himself in any conflicts."
Ultimately, Judge Lee found there was no reasonable possibility that the attack could have been prevented by law enforcement agencies.
"The intelligence gathered did not allow for the details or participants of this act to be identified," the coroner said.
Four men - Raban Alou, Milad Atai, Mustafa Dirani and Talal Alameddine - have been sentenced for their roles in providing a revolver to Farhad.
Alou and Atai were under police surveillance as part of a terrorist group allegedly planning an attack at another Sydney location.
Alou, Atai and Dirani sourced the firearm from Alameddine.
The handover of the gun from Alou to Farhad in an area of Parramatta Mosque where there were no CCTV cameras made obtaining prior knowledge of the attack impossible, the coroner found.
"The final steps of the planned act were only taken about three hours before the shooting," Judge Lee said.
Neither Mr Cheng's family nor Farhad's family were at the NSW Coroners Court when the findings were handed down.
Alpha Cheng told reporters on the first day of the inquest there were two tragedies from the attack.
"One for ourselves and the family losing dad in such a way," he said.
"But also the tragedy of someone who's 15 years old being involved and radicalised and having the ability to do such an act of terrorism."
Alpha described his Chelsea Football Club-loving father as a kind, generous and humorous man during the inquest.
Mr Cheng was posthumously awarded a commendation for outstanding duty by the Police Commissioner and the headquarters where he was slain were renamed in his honour in 2016.
Mr Cheng's killing has "left an indelible impact on his family and the community," Judge Lee said.
He found the actions of the special constable, who cannot be identified for legal reasons, who fired his weapon at Farhad were justified and likely prevented further injuries or deaths.
Programs developed to further identify and counter radicalisation among school students have been implemented in the state since the attack.
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