A man has told US investigators he "felt responsible" for the Wieambilla shootings in Australia that claimed six lives including two police officers, the FBI says.

Brothers Nathaniel, 46, and Gareth Train, 47, used high-powered rifles during an ambush to kill Constables Matthew Arnold, 26, and Rachel McCrow, 29, at the remote Queensland property in December 2022.

Arizona man Donald Day Jr, 60, was charged a year later in the US with making threats to public figures and FBI agents, as well as with illegal firearms possession.

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Prosecutors on Wednesday submitted an FBI report to a pre-trial hearing in the US Federal District Court of Arizona.

The report alleges Day provided information to two FBI special agents during an electronically recorded interview at a hotel in Heber, Arizona after his arrest.

"Yeah, this does have to do with Queensland," Day allegedly told his wife after reading his arrest warrant.

Nathaniel Train joined Gareth and his sibling's wife Stacey, 45, to fatally shoot neighbour Alan Dare, 58, soon after killing the two constables.

All three Trains were shot dead by specialist police officers hours later when they refused to negotiate or surrender.

Day allegedly told the FBI agents a year later: "I totally felt responsible for that … because I didn't want to see my friends get killed like that".

Day allegedly sent messages about a "Christian end-of-days ideology" known as premillennialism to the Trains between May 2021 and December 2022.

An inquest into the shootings was told the Trains had paranoid beliefs police were "devils and demons" who would transform the family into mindless slaves.

The FBI report claimed Day told agents he had "added fuel to the fire", after Queensland Police tried to contact the Trains, instead of dampening the coals.

Day allegedly agreed with agents that he told the Trains prior to the shooting that he would kill police if they entered his property.

"I would kill them all because they are monsters and demons in the flesh," Day allegedly told the Trains via online messages.

Day allegedly told the agents he "did not know that things were going to end the way that they did".

"If he had known, he would have asked the Trains to be patient so he could have the Trains move to the US," the FBI report stated.

Queensland Police visited the front gate on the Trains' property prior to the shooting as part in relation to a missing persons report.

Four junior officers days later returned and entered the property to arrest Nathaniel Train over firearms charges.

The Train brothers opened fire on them from concealed sniper positions.

Day has pleaded not guilty to all charges and denied having any involvement in the Wieambilla shootings.

The trial is scheduled to begin on April 22 in Arizona.

A US judge is due to rule on a motion by Day's lawyer to block Queensland police from testifying, on grounds it would prejudice the jury to hear their "emotionally charged testimony".