A U.S. citizen has been charged in Arizona over online comments that allegedly incited what police describe as a "religiously motivated terrorist attack" in Australia a year ago in which six people died, officials said Wednesday.

Queensland state 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 last Dec. 12, investigators say.

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. Two officers managed to escape and raise the alarm.

Police killed the three Trains, who have been described as conspiracy theorists, during a six-hour siege.

The siege involved "many weapons" and continued for hours, before the suspects were shot by specially trained officers, authorities said, the BBC reported. Investigators say the attack was premeditated, and that it involved "advanced planning and preparation against law enforcement."

The BBC reported that camouflaged hideouts, barriers, guns, knives, closed-circuit TV cameras, and mirrors on trees were set up throughout the property.

FBI agents arrested a 58-year-old man near Heber Overgaard, Arizona, last week on a U.S. charge that alleged he incited the violence through comments posted online last December, Queensland Police Assistant Commissioner Cheryl Scanlon said at a joint news conference in Brisbane with FBI legal attaché for Australia Nitiana Mann. Police did not release the suspect's name.

A search warrant was executed near that Heber Overgaard property that was part of the investigation, CBS affiliate KPHO-TV reported.

The suspect was remanded in custody when he appeared in an Arizona court on Tuesday. He faces a potential five-year prison sentence if convicted.

"We know that the offenders executed a religiously motivated terrorist attack in Queensland," Scanlon said, referring to the Trains. "They were motivated by a Christian extremist ideology."

It is the first time an extreme Christian ideology has been linked to a terror attack in Australia, authorities said, according to the BBC.

The FBI is still investigating the alleged motive of the American. Queensland police had flown to Arizona to help investigators there.

"The attack involved advanced planning and preparation against law enforcement," Scanlon said.

Gareth Train began following the suspect on YouTube in May 2020. 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.

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," she added.

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