Three of four men linked with Parramatta shooting have been released without charge in a move police admitted Thursday was “incredibly frustrating”, according to The Australian.

A number of people were arrested after raids in Sydney linked to a shooting outside a police station.

 Curtis Chen was gunned down outside Parramatta Police Headquarters on Friday by 15-year-old Farhad Jabar, who was then shot dead by police.

Jabar, an Australian born in Iran and of Iraqi-Kurdish heritage, is reported to have shouted religious slogans as he killed Mr Cheng, a police accountant.

Arrested people included past and present students of Arthur Phillip high school, the school attended by Jabar.

Sydney Morning Herald report suggests that a Middle Eastern crime gang is being investigated in relation to supplying the gun used by Jabar. Police believe, High school student Farhad Jabar was recruited by a group of western Sydney extremists to carry out a terrorist attack because they thought they were under too much surveillance to do it themselves.

Australia concerned over younger Australians being targeted and groomed by radicals

Australian Prime Minister Malcolm Turnbull has said the attack “appears to have been an act of terrorism”.

Federal Justice Minister Michael Keenan said on Thursday it was a great concern that younger Australians were being targeted and groomed by radicals “in the same way a paedophile would groom a victim”.

“They seek to identify vulnerable people… to carry out a terror attack in here in Australia,” he told the Seven Network on Thursday.

Immigration Minister Peter Dutton has urged Australians of all religions to have an “honest discussion” about violent extremism, as he warned some young people were being radicalised online in just two or three weeks.

Mr Dutton confirmed immigration officials were looking at the history and visa applications of 15-year-old Farhad Jabar, who shot dead police employee Curtis Cheng last week, as well as his family members.

He said Jabar, born in Iran to Iraqi-Kurd parents, appears to have arrived in Australia with his siblings sometime after their relatives.

“We need to recognise that a lot of these kids have been radicalised online over a couple of weeks, whether or not they’re of the Muslim faith or another faith,” he said on 2GB radio.

“We know they’re receiving this education, this indoctrination online, and many of them wouldn’t have an idea about the values of Islam or about the values of Christianity or anything else.