The investigation into the recruitment of a schoolboy to carry out a terror attack on Parramatta police headquarters is centring on an 18-year-old who has openly expressed his anger at what he believes is a "war on Islam" in Australia.

Fairfax Media understands that police are close to confirming that the Wentworthville teenager, who cannot be named, was the one who sourced a handgun from a Middle Eastern crime figure and handed it to Farhad Jabar in Parramatta Mosque last Friday. 

The 18-year-old was one of four men detained on Wednesday while counter-terrorism police raided their family homes and seized hundreds of items for the investigation into the murder of police accountant Curtis Cheng.

Three of the four men were released on Thursday however police successfully applied for another warrant to place the 18-year-old on an extended detention order.

Such orders can be granted by a magistrate during terrorism investigations that are complex, requiring suspects to be detained for longer periods.

NSW Police Commissioner Andrew Scipione said it was "incredibly frustrating" that three men were released on Thursday but the law was clear in determining when a suspect could be held in custody.

"Much happens during the course of arresting them. We've searched properties, we've seized material, we've seized equipment," he told 2GB Radio.

"We do a whole range of things that are very useful in the ongoing investigation... 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."

The 18-year-old from Wentworthville was targeted in last year's Appleby raids and he lashed out at the time.

He told Fairfax Media he believed he had been targeted for being Muslim.

"I dunno, I got a lot of anger," he said. "It's a war on Islam just because we grow our beards. They want to label us as a terrorist, or supporters of IS, whatever, that's up to you."

It is believed one of the family's associations, a Middle Eastern organised crime figure, sourced a gun for the 18-year-old, who then gave it to Jabar last Friday.

Jabar put it in his backpack along with a rambling note that said "blood will run into the streets" and "know your security means nothing to us".

Jabar had not been on any intelligence radars before then and friends said he turned distant in recent weeks, wagging school to attend Parramatta Mosque and spending more time with a 16 year-old classmate at Arthur Phillip High, an Islamic State supporter.

Mr Cheng's family again expressed their gratitude on Thursday "for the overwhelming sympathy and support from the public, the government and Mr Cheng's police force colleagues".

Clarification: The original version of this story said the 18 year-old man was placed on a Preventative Detention Order. He was actually placed on an additional period of detention under Section 23DD of the Crimes Act.

NOTE: An earlier version of this story was changed for legal reasons.