The shooting deaths of six people including two Queensland police at a rural property two months ago was the nation's first domestic terror attack inspired by "Christian extremist ideology", with a US man a person of interest in the case.
Constables Matthew Arnold and Rachel McCrow were shot dead by Nathaniel, Gareth and Stacey Train after the officers arrived at their Wieambilla property, more than 300km west of Brisbane, on December 12.
Alan Dare was also gunned down after going to check on the commotion, with the Trains killed in a gunfight with specialist police later that night.
Deputy Police Commissioner Tracy Linford said police don't believe the shootings were random or spontaneous but rather a deliberate and premeditated act of terror inspired by extremist Christian beliefs.
"Nathaniel, Gareth and Stacey Train acted as an autonomous cell and executed a religiously motivated terrorist attack," she told reporters on Thursday.
"What we've been able to glean from that information is that the Train family members subscribe to what we'll call a broad Christian fundamentalist belief system known as premillennialism."
Premillennialists hold an apocalyptic belief that the world will go through a period of calamities before the second coming of Jesus Christ.
Investigators found six guns, three compound bows with arrows and a number of knives at the Train property.
Investigators believe no other people in Australia were involved in planning or carrying out the attack, Ms Linford said.
However, a man with an American accent who called himself Don and posted videos online referring to the Trains before and after the shootings is a person of interest.
The FBI has been provided with information about certain people in the US who interacted with the trio on social media.
The unprecedented attack came six months after ASIO director-general Mike Burgess warned right-wing extremism was taking up almost half of his agency's counter-terrorism workload.
"Christian extremist ideology has been linked to other attacks around the world but this is the first time we've seen it appear in Australia," Ms Linford said.
The deputy commissioner said a number of events had pushed the trio towards extreme religious beliefs including Nathaniel Train's heart attack, which was "a profound moment for him and his belief in God".
He and Stacey Train losing their school jobs after refusing to comply with COVID-19 vaccine mandates had also hardened their anti-government views.
Ms Linford said it was possible the trio were hoping to inspire copycats with social media posts and believed they were preparing for the "end of days".
The trio set up a camouflaged hide-out at the property, where one of them would periodically "lie in wait", along with multiple barricades including dirt mounds, logs and steel fences.
There was also dated CCTV footage, radios and mirrors on trees, which police believe the trio used to spot passing cars, and a trapdoor under the house possibly installed for an easy escape.
Investigators are still looking into whether mental health played any part in the attack, as is often the case in lone-wolf terrorist attacks, although this would be unusual if it was a factor for three different people.
"When you've got three acting together, it's challenging to say that it was a mental health issue in this instance, particularly when we look at all the material we've examined," Ms Linford said.
