"A religiously motivated terrorist attack." What we know about the Queensland cop killers.

The shooting deaths of six people including two Queensland police at a rural property two months ago has been declared the nation's first domestic terror attack inspired by "Christian extremist ideology".

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.

Neighbour 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.

Watch: Queensland's police commissioner on the shooting. Story continues after video.

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, 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," 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.

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," Linford said.

Here's everything else we know about the three people behind one of Queensland's deadliest police shootings.

The brothers' evangelical upbringing.

Gareth and Nathaniel were raised in southern Queensland alongside their two other siblings. Their father, Ronald Train, was a Baptist pastor and founded the Christian Independent Fellowship of Toowoomba in 1998.

Speaking to A Current Affair after the shooting, Ronald said that Gareth proved difficult to handle as a teenager and ran into trouble at multiple schools.

"He was very volatile, very controlling," Ronald said.

The men, who Ronald said shared a close relationship as children, both cut ties with the family in their early 20s.

"We tried to find the answers within ourselves, whether we were responsible — we weren't," he said.

"We had to eventually in the end say that they have made this decision as adults and we couldn't do a thing about it."

The first Ronald learned about his sons' involvement in Monday's shooting was when police knocked on his door the following morning.

"I cannot give you a response to why they have done what they have done. They have to be accountable for what they have done," he said. "They will be brought to justice before God."

Stacey married both brothers.

During the A Current Affair interview, Ronald Train revealed that while Stacey Train was married to Gareth at the time of her death, she had previously been married to Nathaniel.

"I married them here in Toowoomba Baptist Church," he said.

Ronald said the couple had two children, who are also estranged from the extended family. 

The retired pastor was unaware that Stacey had since married Gareth, but speculated that it was a result of his "overpowering" elder son having "taken over the relationship" between her and Nathaniel.

According to The Guardian, the trio all lived at the same Cairns address in 2004, and came back under the one roof recently when Nathaniel joined his brother and ex-wife at the property in Wieambilla.

Nathaniel had left his job as the executive principal at Walgett Community College School in NSW after suffering a heart attack in August 2021. 

His family had not seen him since that December but remained in contact until October this year. 

The sudden silence prompted a missing persons report to Walgett Police, which led officers to the Wieambilla property on Monday.

Conspiracies and concerning behaviour.

The Guardian reported that Gareth Train was active on anti-authoritarian websites and forums, where he espoused views that the Queensland Government was running "re-education camps", and that 1996 Port Arthur massacre was a "false flag" operation staged by the government to "disarm" its citizens.

The former social worker also expressed criticism of SERT — the tactical response team that attended the siege.

"If you are conservative, anti-vaxx [sic], freedom lover, protester, common law, conspiracy talker, alternative news, independent critical thinker, truther, Christian, patriot etc etc expect a visit from these hammers," he said.

In one post, Gareth appeared to indicate that he was a survivalist, and had fortified his home in preparation for "a time like no other".

Stacey Train had reportedly also expressed conspiratorial thinking about COVID-19 vaccinations to her colleagues, though had minimal presence online. The former teacher left her job as the head of curriculum at Tara Shire State College on December 16, 2021 — one day before a state government mandate came into effect requiring all education staff to have received their first dose of the COVID-19 jab.

Last month, Stacey and Nathaniel Train biological daughter, Madelyn said there was nothing to indicate the trio would commit the mass shooting but her "dad", Gary, did hold extreme views.

"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 told 9News. 

The 26-year-old said the three were likely "influenced by fear of the unknown" in committing the shooting.

She said she knew her biological father Nathaniel owned guns and that her parents, Stacey and Gary, had a gun safe and licences, but her mother "didn't like guns".

#EXCLUSIVE: The daughter and niece of the rural Queensland shooters that killed two young police officers and a good samaritan reveals what she believes led to the deadly day and how she now has to live with the actions of her family. @sophie_walsh9 #9News pic.twitter.com/3Y36cqubLI

— 9News Australia (@9NewsAUS) January 22, 2023

Madelyn also said her family "were the gentlest people I know".

"I mourn six people; so I mourn my family, I mourn the police and mourn the neighbour," she shared. 

"Grieving three of your family is hard enough but then finding out what they did and seeing everyone's reaction to what they did."

She said she had not seen the trio for years but communicated with them via email and texts.

- With AAP. 

This article was originally published on December 15, 2022, and was updated on February 17, 2023.

Feature Image: 9News/Facebook@ Queensland Police Union of Employees/NBC News/Mamamia.