Two Australian men behind a shootout that left six dead were “obsessed” with guns in their youth, their father says.

Nathaniel and Gareth Train, along with Gareth's wife Stacey, were killed by police on Monday after they shot two police officers and a neighbour.

Police are investigating whether they set an ambush at the remote property, and their links to conspiracy theories.

On Thursday Home Affairs Minister Clare O'Neil said radicalisation was likely to have played a role.

“Radicalisation is not new. But... conspiracy theories, disinformation and misinformation - problems as old as time - are being turbocharged by technology into terrible acts of violence,” she told parliament.

“They are presenting a new kind of threat to our national security.”

Speaking for the first time since the shooting, Ronald Train said his sons were not “monsters” but had “completely lost their way in life” since cutting off their family 20 years ago.

In an exclusive interview with local television programme A Current Affair, Mr. Train said the pair had long shown an interest in weapons.

“Gareth in particular was obsessed... Nathaniel to a lesser degree,” he said.

He described Gareth - the older of the two brothers - as a “difficult” child who was “very volatile [and] very controlling”, but said he couldn't fathom how his sons had come to be involved in such a “horrific” incident.

“I just could not understand how something like this could have occurred with two children who had been raised… with certain beliefs, Christian beliefs,” Mr. Train said.

He also spoke of his surprise at learning Stacey Train, 45, had left her first husband and the father of her children - Nathaniel, 46 - for his brother Gareth, 47.

Police have said the trio used “many weapons”, but are yet to reveal what kind or a motive for the attack.

Officers were at the property in Wieambilla - 270km (168 miles) west of Brisbane - searching for Nathaniel Train, who had been reported missing, when they were fired upon about 16:30 on Monday.

Two constables - Matthew Arnold and Rachel McCrow - were killed. They were hit immediately then shot again “execution-style”, the Queensland Police Union head claims.

A third officer was shot and injured, but escaped, and the fourth faced an hours-long ordeal as the suspects attempted to hunt her down, lighting fires to try and flush her out.

At some point 58-year-old Alan Dare, a neighbour concerned a house was on fire, turned up at the property and was killed by the suspects too.

'Deep and important questions'

Several media outlets reported that Gareth Train appeared to have contributed often to online forums which promoted conspiracy theories. In posts he had expressed a distrust of police and wrongly claimed the country's deadliest mass shooting was a government ploy to disarm Australians, The Guardian reported.

Australia will have to confront “deep and important policy questions” in the wake of the shooting, Ms O'Neil said, but “today is a day for grieving”.

Local MP and Nationals Party Leader David Littleproud, who grew up in a town near the scene, said the shooting had rocked his community “to its core”.

The slain officers were “young people serving their community - a rich and full life ahead of them”, Prime Minister Anthony Albanese said, and Mr Dare was an "innocent Australian who paid for his kindness and concern with his life”.

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