Donald Day, 58, was arrested in the United States after a year-long investigation into his contact with Gareth, Nathaniel and Stacey Train before the trio killed two police officers and a neighbour at their rural Queensland property.
Day faced the US Federal Court in Arizona on Friday AEST after making a deal with prosecutors in October 2025 to plead guilty to a charge of possessing firearms as a convicted felon.
His prior charges of threatening World Health Organisation figures and FBI agents as well as possessing an illegal shotgun were dropped.
Day used the name "Geronimo's Bones" to exchange comments and videos on YouTube with the Trains between May 2021 to December 2022, US District Judge John Tuchi heard.
Brothers Nathaniel, 46, and Gareth, 47, used high-powered rifles during an ambush to kill Constables Matthew Arnold, 26, and Rachel McCrow, 29, at Wieambilla in December 2022.
Gareth Train's wife Stacey, 45, and the brothers fatally shot neighbour Alan Dare, 58, soon afterwards.
Day was mentioned in the last video the Trains ever made, filmed before they were killed in a shootout with tactical officers hours after the Wieambilla ambush.
"They came to us, and we killed them," Gareth and Stacey Train said in the video.
"We'll see you when we go home. We'll see you at home, Don. Love you."
Day responded to the video saying: "those bastards will regret that they ever f***ed with us".
Queensland Police officers had been due to testify at Day's now aborted trial about the alleged serious danger the American's threats posed due to his association with the Trains.
Day had prepared a sniper's nest for a confrontation with police and told the Trains to do the same with "determination and fury", saying he looked forward to "taking the scalps of our enemies".
Day encountered the Train family through public comments and videos posted around a "similar, distrustful view of society".
A Queensland inquest in November 2025 found the Trains were delusional and believed "war had reached their gates" in the form of a battle to the death with Satanic entities disguised as police.
"It was the Trains' shared psychotic disorders, not their communications with Mr Day, that explained their actions," Day's lawyer, Jon Sands, told the US court.
Mr Sands called for his client to be sentenced to two years and three months with 12 months of supervised release.
Prosecutor Timothy Courchaine asked for a sentence of three years and 10 months with three years' supervised release.
Day was sentenced to 36 months' imprisonment minus time already served.
He had spent two years and three months in custody since his December 2023 arrest by the FBI in Arizona.
Day will be on supervised release for three years after serving at least 90 per cent of his term.
He was also ordered to forfeit his firearms and ammunition, including military-style rifles, shotguns and handguns.
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