By the time the operator had time to process those chilling words, Curtis Cheng, a 17-year veteran of the police finance department, had been shot in the head at point blank range.

His killer, a 15-year-old boy dressed in black robes, also lay dead nearby after he was fatally wounded by one of three special constables at the scene.

That teenage assassin, Farhad Khalil Mohammad Jabar, is listed online as being a student at Arthur Phillip High – which stands just 300 metres from where his life ended in a hail of bullets.

As the sobering realisation began to sink in on Saturday that terrorism had arrived at the doorstep of the state's police headquarters, NSW Police Commissioner Andrew Scipione revealed there had been no specific warning of the attack and that Farhad had never previously strayed onto the police radar.

As detectives began to delve into the child's background, it emerged that his journey to police headquarters on Friday had included a visit to Parramatta Mosque, an open prayer hall located approximately one kilometre from the murder scene.

While police will be seeking answers as to how long Farhad spent on the premises and what he was doing, it is understood that he stepped out of his regular clothes at the mosque and changed into the long, dark shirt and pants he was wearing when he carried out the attack.

Farhad's other movements on the way to police headquarters remain unclear.

However, one fact is certain: When he arrived at Charles Street late on Friday, it was a lot less busy than usual.

"It was not your normal Friday. It was remarkably quiet," said a police source. "There were surprisingly very few people on the street because of the long weekend.

"The school holidays are on, a lot of people had left the city to go on holiday and a lot of other people just took the Friday off. It was fortunate."

In the minutes prior to pulling his trigger, Farhad was observed pacing up and down Charles Street, and in the immediate vicinity of the police centre. Onlookers then watched on in horror as the killer strolled past one female police officer before shooting Mr Cheng, at point blank range, in the back of the head.

Parramatta real estate agent Edwin Almeida said he saw someone dressed in a black gown yelling and wielding a gun outside the police building, moments before shots sounded and a man was seen lying on the ground.

In the pivotal moments after Mr Cheng had been slain, Farhadthen became what police have described as "highly agitated".

CCTV footage appears to show him shouting or yelling something out loud. Far from looking to flee the scene, he began moving in a northerly direction up Charles Street before doubling back – where he was confronted by three special constables who had dashed from the police foyer, with guns drawn.

Mr Scipione confirmed: "A number of special constables came out of the building and as they've emerged, they've come under fire."

Mr Almeida, meanwhile, explained how his frightened real estate staff had fled back into their office, adding that in the time it took to shift from the "door to the window", another three or four shots had been fired.

"We looked out the window, saw security guards and what appeared to be a plain-clothes police officer with guns drawn and pointing at the person that was now lying on the floor surrounded by a pool of blood," he said.

"The individual was lying face down on the footpath, directly in front of the childcare centre ... next to the police headquarters. I'm hoping and praying that none of those little ones saw anything."

A video has since emerged that captures the seconds immediately after the gunman was killed. It shows two constables standing over the dead body of Farhad with their guns drawn. A third constable then appears to join them.

As news began to filter across the nation about the double killing outside the NSW police headquarters building, the adjacent childcare centre, neighbouring businesses and surrounding streets were all forced into lockdown.

On Saturday morning, detectives descended upon Parramatta mosque where they were seen removing seven bags of evidence and at least one car with what appeared to be bullet holes in it.

NSW Forensics officers, meanwhile, swept the 150-metre-long State Crime Command street frontage for evidence as the street remained blocked off to the public.

As the day wore on, a fire crew arrived at the scene and deployed high pressure hoses to wash down the bloodstained path. In turn, uniformed police officers came and placed flowers at the same spot with a card signed by staff at Merrylands Police Station.

When Charles Street was finally reopened to the public, residents began returning to their homes, having been forced to stay with friends or relatives the previous night.

With her grandson nervously peering at the police tape that had been blocking off the road, one of those returning residents said she had been left "shocked" when she had heard the age of the shooter. "He is only 12," she said pointing to her grandson, adding: "It's scary to think the boy who shot dead a police officer is only three years older."