Plain-clothed police visited the western Sydney mosque where teen gunman Farhad Khalil Mohammad Jabar prayed several months before the radicalised schoolboy shot dead a NSW Police employee.

The mosque came to the attention of police and intelligence agencies after it emerged a number of men arrested during Sydney's counter-terrorism raids last year were linked to a street darwah group that also prayed at the Parramatta Mosque.

Police are now looking very closely at the associations 15-year-old Jabar made at the mosque before he walked up behind NSW Police accountant Curtis Cheng and shot him in the back of the head on Friday.

Detectives have been given a month's worth of CCTV footage from the Marsden Street mosque, where Jabar is believed to have visited before he carried out Friday's shooting.

The teenager, from North Parramatta, was captured on video waving a gun above his head before special constables emerged from police headquarters and shot him on the footpath.

Counter terrorism police and the NSW Homicide Squad have spent the past three days retracing the teenager's steps leading up to Friday afternoon's shooting, including where he obtained his older-style weapon.

"I'm not aware of any solid evidence at this stage as to where the gun came from but it is certainly a very strong line of inquiry," NSW Police Deputy Commissioner Nick Kaldas said on Monday.

He said he couldn't say for certain what was on the young man's mind as he turned up with his weapon drawn outside the state's policing hub but "clearly he was disturbed".

Fairfax Media understands police and security intelligence agencies were invited to attend the mosque to listen to sermons with plain-clothed officers attending last year.

It comes as the mosque and Hizb ut-Tahrir denied claims one of the imams who gave a speech on Friday was part of the Islamic organisation.

"The alleged links of Farhad Jabar to Hizb ut-Tahrir are so groundless and absurd, they deserve naught but derision," the group said on Monday in a statement.

It is unclear how long Jabar had been praying at the mosque before he carried out Friday's attack however someone who crossed paths with the teen there described him as a "lonely boy from a troubled family".

Parramatta Mosque chairman Neil El-Kadomi dismissed reports the high school student might have sourced the gun from the prayer hall.

"How can he get a gun when we have a camera or four cameras in every corner of the mosque?" he said.

Mr El-Kadomi said he hadn't handed any of the speeches from last Friday to the police but would speak to the imams at this week's sermon.

While Jabar's face was familiar, Mr El-Kadomi said: "if somebody walk in and walk out, he is just like a shadow."

Detectives also asked the chairman about a black backpack, in which Jabar carried his older-style handgun, before dumping it when he left the mosque.

Police later found material – believed to be packaging with branding linked to an Sydney islamic bookstore – inside the backpack.

When asked if they knew the teenager, an employee at the Lakemba Islamic Book Store said "we don't talk in politics, this is an Islamic book store" before hanging up.

The phones at similar bookstores in Sydney went unanswered.

Investigators are still trying to trace the whereabouts of Jabar's sister, who flew out of the country last week, while the rest of Jabar's family has been cooperative.

At least seven of the men detained in counter-terrorism raids last September were linked to the street darwah group, which Islamic State recruiter Mohammad Ali Baryalei played a well-documented role in.

"There is no suggestion that Parramatta mosque is a problem in terms of its leadership, but it would be a happy hunting ground for recruiters," anti-terrorism expert Professor Greg Barton said.