An American citizen has been charged in Arizona with making threatening online comments that followed what police describe as a “religiously motivated terrorist attack” in Queensland a year ago, in which six people died.

Queensland police officers Rachel McCrow and Matthew Arnold and innocent bystander Alan Dare were fatally shot by Gareth Train, his brother Nathaniel Train and Nathanial’s wife, Stacey Train, in an ambush at the Trains’ remote property in the rural community of Wieambilla on December 12, 2022.

Four officers had arrived at the property to investigate reports of a missing person. They walked into a hail of gunfire, police said at the time.

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Two officers managed to escape and raise the alarm.

Police killed the three Trains during a six-hour siege.

The FBI said agents arrested 58-year-old Donald Day Jr near Heber Overgaard, Arizona, last Friday.

Authorities said a federal grand jury indicted Day on two counts of interstate threats.

The indictment said that from about the beginning of 2022 until February 2, 2023, Day “engaged in a course of conduct demonstrating a desire to incite violence and threaten a variety of groups and individuals including law enforcement and government authorities”.

Day, who lives in the northeastern Arizona town of Heber, was remanded to custody after he appeared in a Phoenix court on Tuesday and faces a potential five-year prison sentence if convicted, according to Arizona prosecutors.

Donald Day Jr. Credit: 7NEWS

The FBI is still investigating Day’s alleged motive.

“The attack involved advanced planning and preparation against law enforcement,” Queensland Police assistant commissioner Cheryl Scanlon said, referring to the Trains.

“We know that the offenders executed a religiously motivated terrorist attack in Queensland. They were motivated by a Christian extremist ideology.”

Australian police allege Gareth Train began following Day on YouTube in May 2020.

Constables Matthew Arnold and Rachel McCrow were shot dead at a remote property in Queensland. Credit: AAP
Stacey, Gareth and Nathaniel Train. Credit: 7NEWS

A year later, they were communicating directly.

“The man repeatedly sent messages containing Christian end-of-days ideology to Gareth and then later to Stacey,” Scanlon said.

FBI legal attache for Australia, Nitiana Mann said the FBI was committed to assisting the Queensland Police Service in its investigation.

“The FBI has a long memory and an even longer reach. From Queensland, Australia, to the remote corners of Arizona,” Mann said.

“The FBI and QPS worked jointly and endlessly to bring this man to justice, and he will face the crimes he is alleged to have perpetrated.”