Alleged terrorists behind the murder of NSW police accountant Curtis Cheng first tried to recruit a 14-year-old boy before settling on 15-year-old Farhad Jabar as the gunman.
The ABC Four Corners program cited police as revealing one member told another that because they were under such close surveillance, they needed a "jahil" - Arabic for an ignorant person - to "kill a kaffir".
Police suspect they were referring to a child.
The program said it understood members of the group first tried to recruit a 14-year-old boy, recently convicted of firearms offences, before settling on Jabar.
He shot Mr Cheng, 58, in cold blood outside the NSW Police Parramatta headquarters on October 2 before he was himself shot dead.
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The ABC said Jabar, a year 10 student at the Arthur Phillip High School in Parramatta, had been recruited by members of a particular group identified by police in the longrunning Operation Appleby investigation.
NSW Deputy Police Commissioner Catherine Burn said Jabar had not been a police target and was not somebody they would have assessed as a threat.
However, those allegedly behind the attack had come to police attention in previous terrorist investigations.
Terrorism consultant Shandon Harris-Hogan said those on the periphery of one terrorist investigation quite often comprised the core of the next investigation.
He said radicalisation wasn't any form of top-down recruiting.
"There isn't usually an evil unknown predatory figure somewhere on the internet," he said.
"It isn't individuals being brainwashed. People aren't radicalised by online propaganda.
"It's fundamentally a social process."
Australian Federal Police Assistant Commissioner Neil Gaughan said most of these people knew each other.
"It's a very small community that are engaged in illegal activity and even a smaller, much smaller group that wish to do Australians harm," he said.
