It was 11am last Friday and the unprepossessing mosque, tucked away on the first floor of an apartment block in Parramatta's central business district, was bustling with activity as clerics prepared for the Jummah, or Friday prayers, due to start some two hours later.

For 40 minutes, 15-year-old Jabar was a study of devout repose, says the mosque's chairman Neil El-Kadomi.

It was hardly an unusual sight. Most afternoons – if he didn't have basketball practice or tutoring – Jabar would pray at the mosque, which is a couple of blocks away from the Arthur Phillip High School, where he was a year 10 student.

If Jabar stayed to listen to the Friday prayers, El-Kadomi said he would have heard a message of charity, of how best to help others, how to worship God and the importance of the pilgrimage to Mecca and Medina for observant Muslims.

But Jabar was in the final throes of a plan to murder a police officer, any police officer, that day. The target was the NSW Police Headquarters, less than one kilometre  away and home to elite crime squads investigating homicides, drug trafficking and serious robberies.

Whether he left the mosque before returning remains unclear. But police believe it was at the mosque that Jabar met with several older teenagers and men later that afternoon, who passed to him a silver .38 Smith and Wesson handgun.

Jabar then changed into black robes and trousers. Worshippers at the mosque who knew Jabar said it was the first time they'd seen him in traditional Islamic dress.

Police believe he then left the mosque with the gun in a black Nike backpack that also contained a note he had written saying "blood will run into the streets".

Jabar took out the gun, dumped the bag outside his high school and walked another 300 metres to the NSW police building.

At around 4.30pm, police accountant Curtis Cheng walked out of the building, heading home to see his family for the long weekend. Jabar followed behind, handgun drawn, and shot him in the head from almost point-blank range.

Within minutes, Jabar too was dead, shot by one of three special constables who guard the building as he fired indiscriminately and shouted religious slogans.

Family, friends and colleagues of the widely admired Cheng are devastated by the sudden and unfathomable loss of a deeply loved husband and father.

For the wider community, familiar and difficult questions have been asked. Why are ever-younger Australians falling under the thrall of extremism and prepared to undertake despicable acts?

Why were police unable to prevent the attacks when Jabar was close to known extremists?

What, if anything, can be done about it?

In Farhad Jabar's case, there are pointers but no conclusive evidence about what motivated him to commit murder.

There are family links. His older sister Shadi – who Jabar was living with – left the day before the killing for Turkey, reportedly en route for Syria or Iraq, where the terrorist group Islamic State has created its blood-soaked caliphate.

Then there are the four young men detained in raids on Wednesday morning, all known to police for their extremist beliefs or caught up in massive counter-terrorism raids last year but later released.

Only one, an 18-year-old, remained in police custody on Friday, after a magistrate allowed an additional period of detention that will keep him incarcerated until Monday while police continue to question him.

The working theory is that the men had long wanted to attack police but felt they were under too much surveillance so instead convinced Jabar, a youth with no criminal record and not on any police or intelligence agency radar, to do it for them.

All had been regular attendees of Parramatta Mosque. Intelligence sources say the mosque's imams are not known for preaching a radical brand of Islam.

Instead, it appears the mosque was being used by a cell of extremists for face-to-face meetings away from electronic eavesdropping by counter-terrorism authorities.

The activities at Arthur Phillip High School are also under scrutiny. The detained 18-year-old was a former student. A 16-year-old – detained and then released this week – was a classmate of Jabar and regularly attended lunchtime prayer services.

The 16-year-old gained notoriety after driving outside a Christian school hurling insults and brandishing an Islamic State flag last year.

Meanwhile, a 17-year-old Arthur Phillip student was detained on Tuesday on his way to school after he was found to have posted remarks on social media supportive of Jabar's murder and saying he hoped that police in Merrylands would be the next target.

Of Iraqi-Kurdish background and born in Iran, Jabar's publicly accessible social media accounts depict the usual preoccupations of a young adolescent. He liked The Simpsons, reality TV talent contest The Voice and US basketball.

On the popular app ask.fm, Jabar reportedly revealed he also liked cats and computer games. Responding to the question what he would do "if he was invisible", Jabar replied: "I would do pranks and scare people".

Peter, a close friend and teammate in the school basketball team said Jabar was a cheerful, smart, private student who never spoke about religion openly at school and was not a ringleader.

"He would go to the mosque but he didn't like talking about religion. He'd rather joke around so it doesn't make any sense what he's done."

However, says Peter, he became distant in the weeks before the end of the last school term.

"He didn't show up to any of our games, he would just disappear," he said. "I asked him what was happening and he just said 'Oh I had to go home to finish off some assignments'."

Numerous students and friends have made the same observation.

According to one friend quoted in The Guardian, he knew something was wrong when – in the company of older men at the mosque – Jabar pushed him away.

"I went over to the group, said to them Salaam Alaikum​, peace be upon you. And he shook my hand, then brushed me off and the rest of the group didn't respond," he said.

"That's a big deal in the Islamic faith. One of the rights you have upon a fellow Muslim is to greet them with the best greeting. But he responded really coldly. I got the sense he didn't want the others to know we had an interaction going on."

A Muslim community mental health worker told Fairfax Media he had observed depressive symptoms in Jabar in recent months and believed he was suffering mental health issues. He has twice been interviewed by the police since Friday.

While the specifics of Jabar's radicalisation are yet to fully emerge, his act of terrorism was part of a broader trend in that he is young and he targeted police.

Twice in the past year, police have been attacked by individuals, apparently inspired by a militant brand of Islam and intent on murder. Moreover, another alleged plot to launch an attack in Melbourne on Anzac Day this year was to begin by running over police officers in a car before beheading them and stealing their weapons.

All those involved in these cases were teenagers. Incredibly, the Anzac Day plot was instigated by a British 14-year-old through online communications.

In May, a Melbourne teenager was caught with three pipe bombs and allegedly planning an attack on Mother's Day. A procession of youngsters have headed to Syria – or made the attempt – to join Islamic State and like-minded groups.

Teens are easy marks for terrorist recruiters.

Scientific research shows that the brains of young people are vastly different from those of adults. The prefrontal cortex, responsible for executive functions which co-ordinate thoughts and action and foresees the consequences, are underdeveloped.

Young people are impulsive, risk takers. At once, they tend to reject authority while craving a sense of belonging.

The twisted but emphatic vision of Islamic State offers both, while also promising an eventful, meaningful life and an even more glorious death.

Moreover, peer influence has an outsized impact on the adolescent brain, activating areas of the brain associated with reward processing in adolescents but not adults.

Indeed, the likelihood of adolescents engaging in risky behaviour actually increases when they are shown evidence such behaviour will be harmful.

"It is a rebellion and it is about something that they see as glamorous, as romantic, heroic as they see it – and wanting to belong to a particular thing," Deputy NSW Police Commissioner Nick Kaldas​ said this week.

Shameema Kolia, a Muslim youth worker involved in the Australian Multicultural Foundation's program to prevent youth becoming radicalised, says it's important to establish trust and a connection with young people displaying antisocial tendencies.

"Once you have that with young people it makes a huge difference. In fact, that's what the [terrorist] recruiters do. They get online and become their best buddies.

"They isolate them from the rest of the community and get to work on them."

"We have to counter that. Young people have to feel safe and they have to feel that they are being heard. If not, whatever you say falls on deaf ears."

Families, schools and – perhaps most importantly – other young people all play a part.

Muslim leaders have called for redoubled actions to weed out extremism. Parramatta Mosque chairman El-Kadomi was to tell worshippers at Friday prayers this week: "If you don't like Australia, leave."

After months of evidence that radical preaching is taking place in high schools, a NSW audit has been delivered but not released.

Of the 20 or so young Australians linked to extremist groups and terrorism plots, the vast majority attended government schools like Arthur Phillip High. A nationwide deradicalisation program for schools begun by former education minister Christopher Pyne is likely to be rolled out shortly.

But the efforts by government have been ham-fisted so far. A radicalisation awareness kit distributed to school teachers used the bizarre example of "Karen", a green activist fond of alternative music, to highlight the risks of violent extremism.

A Defence Force twitter account promoting anti-Islamic State messages was shut down after it was found to be repeatedly mistranslating its messages into Arabic.

Missing in Australia is a compelling counter-narrative for young people vulnerable to radicalisation born of real-life experience.

The tragic and pointless death of Melbourne teenager Jake Bilardi, killed in a suicide bombing attack that failed to hit its target, is perhaps one example.

Other young Australians who have joined Islamic State or been to Syria and Iraq and become disillusioned have yet to be heard. Some have tried to return, others hope to but can't due to tough anti-terrorism laws.

"There's resistance in government and among the police," says Kolia. "They are worried that these people might [inadvertently] give people tips on how to get over there.

"I think the police need to reanalyse it. If no one is talking about what's really going on over there, it's still going to seem like a utopia. We need to show the reality of these groups in very strong terms."

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