Three of four males arrested during a series of dawn raids across Sydney on Wednesday have been released without charge, as NSW Police continue to investigate last week's execution outside its Parramatta headquarters.

Only an 18-year-old man, from Wentworthville, remained in custody on Thursday morning in connection with the killing of civilian police employee Curtis Cheng, who was shot dead on Friday afternoon. No charges have been laid.

NSW Police Commissioner Andrew Scipione said it was "incredibly frustrating", but said the law was quite clear in determining when a suspect could be held in custody.

He told Alan Jones on radio station 2GB on Thursday that investigators were ploughing through a large amount of information and equipment seized on Wednesday morning when about 200 officers raided four homes in Sydney's west and north.

"Only when we have sufficient evidence that can put us in a position where we can charge an offender with an offence, put them before a court, that we can hold them," Mr Scipione said.

"Much happens during the course of arresting them. We've searched properties, we've seized material, we've seized equipment.

"We do a whole range of things that are very useful in the ongoing investigation, but again, when we've got sufficient evidence or intelligence to get a warrant, we actually execute the warrant and then we're looking to go to the next stage.

"If we find evidence at the time, we use it. We've taken a lot of material during the course of these searches and that's going to take us a long time to go through."

Mr Scipione said there was "no denying" that the 15-year-old gunman Farhad Jabar, who shot Mr Cheng about 4.30pm on Friday before he himself was killed, had been radicalised.

He said that, within 12 hours of the shooting, police knew that Jabar, a year 10 student at the nearby Arthur Phillip High School, had terrorist links.

"Certainly we know that this young man had been radicalised. We know that now," he said.

"[Initially] we had no understanding of what his motives were. Certainly [police] didn't know if he was acting alone or in concert, but that was only because we didn't know who he was.

"It didn't take us long, though. Within six to 12 hours, we clearly identified he had links, and I think a bit over 12 hours after my first press conference, I simply said this man has terrorist links. Let's call it as it is ... we can't defy gravity."

Mr Scipione said that, due to the ongoing investigation, he could not comment on what was found in Jabar's backpack that was found dumped near the shooting scene, nor whether the teenager had any links to a known British radical associated with Islamic State.

Fairfax Media understands that investigators found a note in Jabar's backpack that said: "blood will run into the streets" and "know your security means nothing to us". The handwriting has been confirmed as Jabar's, Fairfax Media has been told.

Mr Scipione said young, vulnerable people were being preyed upon by radicals around the world, and the radicalisation process occurred very quickly.

"These young people that are preyed upon - and they are preyed upon by violent extremist predators, it's the only way you can describe them - are then involved in these really horrible attacks with no justification," he said.

"I think at the stage that they're being preyed upon they're victims, but the minute they pull the trigger, they go from being a victim to a murderer. That's reality."

He described Mr Cheng, an accountant with the police force and father-of-two, as "almost the Richie Cunningham of the NSW Police Force", referring to the Happy Days character played by Ron Howard.

"He was such a nice gentleman," Mr Scipione said of Mr Cheng.

"To go and hug his wife on Saturday morning, in the midst of all of the tears ... you realise that this isn't a Hollywood set. This is real life, real life Australia."