Police investigating the execution of one of their colleagues at their Sydney headquarters want to know how the teen gunman was radicalised, and where he gained access to the firearm.

Questions remain whether Farhad Jabar Khalil Mohammad acted alone or had support before he shot NSW Police employee Curtis Cheng dead on Friday.

Curtis Cheng (far left) with wife Selina, daughter Zilvia and son Alpha. (Supplied)

Farhad Jabar. (Image: Instagram)

Jabar visited a nearby mosque before carrying out his attack. The mosque has been subjected to online vitriol since the shooting and today called for calm.

"We understand that a tragedy occurred... however that does not give anyone the right to spread hate against Muslims," the mosque said on their Facebook page.

Foreign Minister Julie Bishop, NSW Premier Mike Baird and Prime Minister Malcolm Turnbull have spoken with Muslim community leaders in the hopes of formulating a plan to prevent the radicalisation of vulnerable teens.

The teen visted a local Parramatta mosque before his attack. (Instagram)

Police are trying to track down the gunman's family.

Reports claim the boy was on the fringe of a radical group which may have encouraged him to carry out the attack.

Police are also trying to track down the teenager's family, some of which, including his sister, left Australia on Thursday, the day before the attack.

The teen's older brother reportedly attended the scene of the shooting yesterday and told police he believed his brother was involved.

NSW Police Commissioner Andrew Scipione said initial investigations suggested the boy's attack was an act of terrorism.

"I can confirm that we have identified that he is a 15-year-old boy," Mr Scipione said.

"We understand he is of an Iraqi-Kurdish background and he was born in Iran.

"We had no information this individual posed this type of threat.