The Federal Bureau of Investigation (FBI) has arrested a man in the United States "in connection to the religiously motivated terrorist attack" that killed three people in Wieambilla in 2022.
The FBI arrested a 58-year-old U.S. national Donald Day in Arizona on December 1, Queensland Police said.
He was remanded in custody in the U.S. and a remote property in Arizona was searched in connection to the investigation.
The Queensland town of Wieambilla in the Western Downs Region lost two police officers and a member of the public during the shooting attack on December 12, 2022.
The three alleged shooters, Gareth Daniel Train, 47, Nathaniel Charles Train, 46, and Stacey Jane Train, 45, were shot dead during the confrontation with police.
How did Gareth Train meet Donald Day?
"We know Gareth Train began following the 58-year-old man on the online platform YouTube around May 2020," Queensland Police assistant commissioner Cheryl Scanlon said.
"Gareth and the man began commenting directly on each other's videos in May 2021," she said.
"Between May 2021 and December 2022 the man repeatedly sent messages containing Christian, end-of-days ideology to Gareth and then later to Stacey.
"The motivation of the United States national is still under investigation by the FBI."
What is premillennialism?
Assistant commissioner Scanlon said the Wieambilla shooters were motivated by a "Christian extremist ideology" called premillennialism.
Premillennialism is a branch of eschatology, a part of Christian theology.
Eschatology focusses on death and judgement, what happens to the soul, and ultimately what happens to the human race.
"There was a belief that Christ will return to the Earth for 1,000 days, and provide peace and prosperity," deputy commissioner Tracy Linford said in February.
"But it will be preceded by an era, or a period of time of tribulation, widespread destruction and suffering," she said.
Premillennialists say there are four stages to human history: tribulation, second coming, millennium and final judgement.
QLD investigation goes abroad
Queensland Police officers travelled to the U.S. to join the FBI search while the Arizona investigation remained ongoing.
Police said two indictments were issued by a grand jury on November 29 in Tucson, Arizona "for interstate threats".
"One of those threats made relates to comments posted online in December 2022 inciting violence in connection with the incident in Wieambilla," she said.
The other was an unrelated threat allegedly made against the head of the World Health Organization.
Police seek to "identify any residual threats posed to the Queensland and Australian community" as a result of the attack, assistant commissioner Scanlon said.
The families of Constable Matthew Arnold, Constable Rachel McCrow, and Alan Dare, a neighbour to the attacker, were notified the arrest had been made.
The State Coroner was also notified.