Mr Haider's family, including an older brother, arrived in Australia from Afghanistan in the mid-2000s, living in Adelaide for several years before moving to Melbourne, according to several media reports.

During high school, the Narre Warren teenager enjoyed cars, sport and did not hold radical views on Islam.

The 18-year-old graduated from Lyndale Secondary College before studying electro engineering at a TAFE college in nearby Dandenong, in Melbourne's south-east.

In early 2013, Mr Haider spent nine days at a Muslim youth camp. His cabin mate, Mohammed Ibrahim, remembered how Mr Haider was enthusiastic about his faith, but said he was not an extremist.

"He was a gentle, soft spoken, young boy," he told ABC's PM radio program. "There was no sign that he was someone that we'd have to be worried about."

But Mr Haider began associating with hardline Muslims at the al-Furqan Centre in Melbourne's outer south-east, which was the focus of counter-terrorism raids in 2012.

On July 25, police and security agencies took notice of Mr Haider's activities following a loud debate at the Fountain Gate shopping centre in Narre Warren.

Police were called to deal with a group of young Muslim men, including Mr Haider, who had been arguing about jihadism and martyrdom, The Australian reports.

But a friend disputes that Mr Haider had become radicalised at that time.

"He was not extreme. He was very well liked in our community," the friend, Maryam Taylor Muhammad said.

In mid-2014, Mr Haider's relationship with a Sri Lankan woman he had converted to Islam soured.

"It (the break-up) was a huge factor in his life," one man who knew Mr Haider told the Herald Sun. "They were living apart. He was very upset."

The teenager, however, was now on the police's radar and being monitored by security agencies.

Fairfax Media has revealed how Mr Haider appears to have developed links with Australian extremists as part of a network connected with Islamic State in Syria and Iraq.

Mr Haider's passport was cancelled by ASIO on national security grounds in mid-September.

Authorities suspected he was planning to leave the country to fight with the Islamic State, News Corp Australia reports.

A friend said Mr Haider felt "harassed" by security agencies and police.

"They always were knocking on his door, asking questions about him," the man told the Herald Sun.

Mr Haider was then seen at Dandenong Plaza last week with an Islamic State flag and was cautioned by police about causing a disturbance.

He also posted a photograph of himself on Facebook holding the flag while wearing a balaclava and military camouflage.

Mr Haider wrote on Facebook: "Let's not put the focus on other things. The main message I'm sending with these statuses and photos is to the dogs of the Australian Federal Police and ASIO who are declaring war on Islam and Muslims."

A close family friend, who did not want to be identified, said Mr Haider's family was becoming increasingly worried about his behaviour and urged him to see a counsellor, ABC television reports. His parents had been looking into an arranged marriage as a way of getting him to settle down.

Mr Haider had also been picked up on intelligence sweeps looking into threats to Parliament House and he may have tried to follow the movements of Prime Minister Tony Abbott, the Herald Sun reports.

But Victoria Police Chief Commissioner Ken Lay sought to downplay those concerns, saying federal police found the link to be "quite tenuous" and police would make a judgment call after reviewing all the evidence.

Counter-terrorism officers stepped up efforts to meet with Mr Haider this week. The plan was not to arrest him, but caution him about his activities, police said.

Police first attended Mr Haider's residence on Tuesday morning, but he was not home. Instead, someone at the house allowed police to conduct a search of his bedroom, Mr Lay said.

The previous day, Islamic State urged followers to kill any Australian with any weapon they could find.

Mr Haider later organised to meet the officers outside the Endeavour Hills police station.

But his mother and father had tried to stop him from leaving that night, citing concerns for his safety, ABC TV reports.

The officers met the teenager in the car park about 7.30pm on Tuesday and, after trying to shake hands, police say Mr Haider pulled out a knife and started stabbing them.

One shot was fired, killing the teenager, while the two officers were taken to hospital.

Mr Lay revealed on Thursday how police are now investigating if the teenager was acting alone at the time.

"On the night, there is some information that would suggest otherwise," he told ABC radio 774 in Melbourne.

"He was certainly talking to other people around the time he came to the police station. It's really unclear as to whether at the time or just before he had other people with him or not, but it's certainly something we're pursuing."