The deadly shootings of two Queensland police officers and a neighbour was Australia's first domestic terror attack motivated by "Christian extremist ideology", with a US man identified as a person of interest in the investigation.
Constables Matthew Arnold and Rachel McCrow and neighbour Alan Dare were shot dead by Nathaniel, Gareth and Stacey Train on their Wieambilla property, more than 300km west of Brisbane, on December 12.
The Trains were later killed in a gunfight with specialist police.
Deputy Police Commissioner Tracy Linford says after taking more than 190 statements and combing through the lives of the Train family, investigators did not believe the attack was random or spontaneous but rather deliberate and premeditated.
"Nathaniel, Gareth and Stacey Train acted as an autonomous cell and executed a religiously-motivated terrorist attack," Ms Linford told reporters on Thursday.
"The Train family members subscribe to what we would call a broad Christian fundamentalist belief system known as premillennialism - it's a belief system that comes from Christian theology."
Investigators believe no other people in Australia were involved in the attack.
However, a man with an American accent who called himself Don and posted videos online referring to the Trains before and after the attack is a person of interest in the case.
Ms Lindford said local investigators have provided information about people in the US to law enforcement agencies there, including the FBI.
"We absolutely believe they act as an autonomous cell but one of our avenues of inquiry is that they did make social media postings and there were people in the US who did monitor those social media postings and make responses to those social media postings," she said.
"So we have worked with our US counterparts, provided the information we have, and they'll determine what investigations they might make as a result of that information."
She said there was no evidence any of the Trains had subscribed or been involved with the sovereign citizens movement, as speculated in the media.
It's the first time a violent terrorist attack motivated by extremist Christian ideology has occurred in the country.
"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," the deputy commissioner said.
She said a number of events had pushed them toward extremist religious ideology, such as 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 due to COVID-19 vaccine mandates hardened their anti-government views.
The trio hung hides, wore camouflaged clothing and erected multiple barriers around the property, with investigators seizing six firearms, compound bows and arrows and knives.
Ms Lindford 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".
Investigators are still looking into whether mental health played any part in the attack, as is often the case in lone-wolf terrorist attacks, but she said it 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 Lindford said.
© AAP 2023