Hundreds of counter-terrorism police conducted dawn raids of homes across Sydney yesterday and arrested four people, aged 16 to 22, believed to be linked to a deadly attack last Friday by a 15-year-old schoolboy.
Two of the men arrested attended the same school in western Sydney as Farhad Jabar Khalil Mohammad, the Iranian-born teenager behind last week's shooting.
The arrests yesterday came as police revealed they believed Farhad was not acting alone, and as possible links emerged between the shooting and a plot inspired by the Islamic State in Iraq and Syria (ISIS) last year to conduct attacks in Sydney.
Farhad went to a mosque service in Parramatta on Friday and then shot dead a 58-year-old police employee, Mr Curtis Cheng, outside a nearby police headquarters before being killed by police guards.
According to a Fairfax Media report yesterday, police believe Farhad was recruited by a group of extremists to conduct the attack because they were under too much surveillance.
New South Wales (NSW) Police Deputy Commissioner Catherine Burn said yesterday that the reasons for the attack remain unknown.
No terrorist organisation has claimed responsibility for it.
"We suspect that a terrorism event has occurred and we suspect that they may have some knowledge," Ms Burn said of the four who were detained.
"We don't really know the motivation. What we would suggest and we suspect is that there was some influence, whether it was ideological, religious or politically motivated, that determined and influenced the 15-year-old to go and commit this horrendous act of violence."
Investigators have asked the authorities in Turkey to assist with a search for Farhad's sister, who flew out of Australia for Istanbul on the day before the attack. It is believed she was planning to travel to Iraq or Syria to fight for ISIS.
"Investigators are very keen to have a chat with her about what she knows about her brother's action," said Australian Federal Police Acting Deputy Commissioner Neil Gaughan.
Police this week searched the Parramatta mosque attended by Farhad but said no link had been established between the mosque and the attack.
Three of the four men arrested yesterday were subsequently released.
More than 200 police officers were involved in the raids.
The homes raided reportedly included the house of a man who was charged last year over an alleged plot by ISIS supporters to conduct various attacks, including a filmed beheading in Sydney.
Those involved in the plot reportedly attended Parramatta mosque.
Ms Burn confirmed the homes raided yesterday were the same as those targeted last year in connection with the plot. But she said Farhad was not known to police until after the attack.
"He was not a threat to us, he was not a target to us," she said. "We can't be watching every single person at every single second."
The revelation that the shooting was conducted by a 15-year-old has come as a shock in Australia and prompted pledges by state and federal political leaders to combat radicalisation of youth.
The New South Wales state Premier, Mr Mike Baird, has asked the Education Department to accelerate efforts to combat radicalisation among school students but insisted the problem was not widespread. "We need to understand that we are in a new world," he said yesterday.
The shooting followed numerous attacks and plots by Islamists in Australia, as well as continuing attempts by young Australians to travel to the Middle East to fight for ISIS.
Ms Lynne Goodwin, the principal of the school attended by Farhad, said in a message on the school's website that NSW police had advised the school that "there is no ongoing threat as a result of last Friday's tragic event".