Some of those arrested in dawn raids across western Sydney on Wednesday morning had attended the same Parramatta mosque where 15-year-old Farhad Khalil Mohammad Jabar spent time on Friday before he shot and killed accountant Curtis Cheng at the nearby police headquarters.
NSW Police Deputy Commissioner Catherine Burn says authorities suspect those arrested have some knowledge of Friday's shooting and could have influenced Jabar.
They included past and present students of Arthur Phillip High School, the same school attended by Jabar, including a 16-year-old who was in the same year.
More than 200 armed officers raided homes in Guilford, Wentworthville, Merrylands and Marsfield at 6am on Wednesday, arresting four males aged between 16 and 22.
At least three of the four were also targeted last September as part of Operation Appleby, the nation's largest counter-terrorism operation.
One of the addresses raided included the Guildford home of Omarjan Azari, arrested as part of Operation Appleby and later charged with conspiring to commit murder and doing an act in preparation for a terrorist attack.
Azari was allegedly acting on instructions from Australia's most senior Islamic State recruit Mohammad Ali Baryalei to kill "a random unbeliever".
Baryalei, who was killed in Syria, was also known to attend the Parramatta mosque. Those detained on Wednesday included an 18-year-old, arrested at a home in Wentworthville where his elder brother was targeted in the raids last September and a Marsfield man.
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Two men, both aged 22, have since been released, a police spokeswoman said. Ms Burn said it was a "very serious concern that in the heart of our community there is attack planning that is underway".
Police were investigating the roles of those arrested.
"Whether or not they inspired it, I don't know at this particular point of time," Ms Burn said.
Ms Burn said authorities were still unsure of Jabar's motivation but suspect "there was some influence" that was either of an ideological, religious or political nature. "We definitely have our suspicion he did not act alone," she said.
"We have some confirmation of some of the people we're interested in attending the mosque. I think it is important though that again it is early days and any substantive link cannot necessarily be established."
NSW Police arrest suspects after raids in Sydney. Source: NSW Police
Police have not confirmed reports the gun used by Jabar came from a Middle Eastern crime figure.
The Australian Federal Police's Acting Deputy Commissioner of National Security, Neil Gaughan, said that while authorities had disrupted six attacks there was "a large number of persons of interest across the country" and "a large number of persons on the periphery of those investigations that we clearly cannot monitor 24/7".
"We will continue to operate I think in this heightened threat environment ... around terrorism matters for a considerable period to come," he said.
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The AFP had been in contact with Turkish counterparts to try to establish the whereabouts of Jabar's elder sister, who flew to Turkey the day before the shooting, and is feared to have been attempting to travel to Iraq or Syria.
"I might stress at this time there is no suggestion that she has been involved in any criminal activity here, but obviously investigators are very keen to have a chat with her about what she knows about her brother's action," Mr Gaughan said.
