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.

In December last year, Queensland police officers Constable Matthew Arnold and Constable Rachel McCrow were murdered on a Wieambilla property, more than 300 kilometres west of Brisbane, by a trio of conspiracy theorists.

Nathaniel, Gareth and Stacey Train were later killed in a gunfight later that night with specialist police.

Deputy Police Commissioner Tracy Linford said investigators had taken more than 190 statements and combed through the lives of the Train family.

She said although the investigation was ongoing, police did not believe the attack was random or spontaneous, and officers had been deliberately targeted.

“Our assessment has concluded that Nathaniel, Gareth and Stacey Train, acted as an autonomous cell and executed a religiously motivated terrorist attack,” Ms Linford said.

“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.”

Premillennialism is described on some Christian websites as a branch that takes literal interpretation of end times, including seeing some events in the Bible’s Old Testament as a “futuristic, progressive chronology”.

Ms Linford said the Trains saw police “as monsters and demons”.

“We do believe it was an attack directed at police,” she said.

“There was significant evidence of advanced preparation and planning.”

Queensland police believe the Trains acted alone, and no other people were involved in the horrifying December 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 had 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.

“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 several events had pushed the Trains toward extremist religious ideology, such as Nathaniel’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.

Police say 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 they 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, 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.

Madelyn Train, the biological daughter of Nathaniel and Stacey, has given several interviews to insist there were no warning signs before the fatal shootout.

However, she admitted Gareth did hold extreme views and was a military enthusiast.

“It was like he was doing this weird project on COVID and the end of the world but also religiously. He believes in the apocalypse,” she said.

Ms Train said she would often try to steer her uncle to other subjects when conversations took a dark turn.

She said she knew Nathaniel owned guns and that Stacey and Gary had a gun safe and weapon licences.

“But mum didn’t like guns,” Ms Train said in the televised interview.

-with AAP