A teen gunman who shot and killed a NSW Police Service employee in western Sydney on Friday acted after he heard a lecture by the extremist group Hizb ut-Tahrir, a local news network has reported. However, the group has downplayed the claim as “speculation”.

Seven News said Farhad Khalil Mohammad Jabar was at a talk given by Hizb ut-Tahrir at nearby Parramatta Mosque just hours before he donned a black robe and killed finance worker Curtis Cheng outside NSW Police headquarters in Parramatta.

It has also emerged that the shooter’s sister flew out of Australia on a flight bound for the Middle East 24-hours before the terror attack.

The gunman has been linked to an Islamist group already under investigation. http://t.co/MitOO7vBxV #7News http://t.co/TYkMqF93HR — 7 News Sydney (@7NewsSydney) October 4, 2015

According to The Sydney Morning Herald, a spokesman for Hizb ut-Tahrir in Australia, Uthman Badar would not comment on the existence of any links between Farhad and his organisation.

“Any links are speculation and we could caution against making speculation,” Mr Badar said.

Hizb ut-Tahrir rejects democracy, secularism and all Western models of state. On Sunday it held a rally in outer Sydney protesting against military action by Russia and the US in Syria.

Parramatta Mosque is believed to have been searched overnight. It sits a few blocks away from the site of the shooting that killed Mr Cheng, 58, as he left work at 4:30pm on Friday.

A senior figure at the mosque has confirmed that police searched the building to look for a black backpack which they believe Farhad used to carry the gun he used to kill Mr Cheng.

Police said the warrant was undertaken by arrangement with leadership at the mosque, who gave their full assistance to police.

Earlier, ABC news reported it was confirmed by a senior police source that the older brother of the shooter tipped them off about the identity of Farhad.

It is also understood Farhad’s sister Shadi went missing on Thursday and flew out of Australia on a Singapore Airlines flight bound for Istanbul and may be attempting to reach Iraq or Syria.

Her family told police she had taken all her belongings.