A United States national sent "Christian end of days ideology" to YouTube followers in Queensland for almost two years, police say.
By December 2022, the same followers were involved in a religiously motivated terrorist attack west of Brisbane that left two police officers and a good Samaritan dead.
Almost a year after the fatal shootings, the US national - a 58-year-old man identified in media reports as Donald Day - has been arrested by the FBI in Arizona.
One of the two indictments issued against him relates to the incitement of violence online in connection to the Queensland attack.
Constables Matthew Arnold and Rachel McCrow were gunned down in cold blood by Nathaniel, Gareth and Stacey Train after the officers arrived at a Wieambilla property on December 12, 2022.
Neighbour Alan Dare was also shot dead before the Trains were killed in a gunfight with specialist police later that night.
Queensland Police on Wednesday said the 58-year-old had appeared in court and was in custody in the US after a joint investigation led to his arrest last week at a remote property near Heber-Overgaard, Arizona.
The man's motivations are still being investigated but it is alleged Gareth Train began following the 58-year-old's YouTube account from May 2020.
The men began commenting on each other's videos a year later, Queensland Police Assistant Commissioner Cheryl Scanlon 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," she told reporters.
The Trains also accessed the man's older YouTube account which was created in 2014, police said.
The Trains subscribed to a broad Christian fundamentalist belief system known as premillennialism.
"We know that the offenders executed a religiously motivated terrorist attack in Queensland," Assistant Commissioner Scanlon said.
"They were motivated by a Christian extremist ideology."
Premillennialists hold an apocalyptic belief that the world will go through a period of calamities before the second coming of Jesus Christ.
Gareth and Stacey Train uploaded a video on a now-deleted YouTube account hours after the fatal shootings, referring to police as "devils and demons" and sending a message to someone called "Don".
There are currently no extradition orders for the 58-year-old man who was charged under US law but Assistant Commissioner Scanlon said it was "early days".
The joint Queensland Police-FBI investigation is continuing after evidence was seized at the Arizona property.
"The FBI has a long memory and an even longer reach," FBI representative Nitiana Mann said in Brisbane on Wednesday.
"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."
The Wieambilla shootings were described as the nation's first domestic terror attack.
Dressed in camouflage fatigues, the heavily armed Trains rigged motion sensors along the entrance to the property to warn them of police arriving.
Constables Arnold, 26, and McCrow, 29, were wounded then fatally shot at close range within minutes of entering the property for a welfare check on a missing person.
Two other officers, constables Randall Kirk and Keeley Brough, escaped under heavy fire.
The Trains lit fires in an attempt to flush out a female officer who took cover in nearby bushland.
Mr Dare, 58, was shot dead when he attended the property to investigate the blaze.
Assistant Commissioner Scanlon said the state coroner and the victims' families had been notified of the arrest.
She said the investigation had a long way to go but described the man's arrest as extremely important.
"This is a terribly tragic event and with the loss of lives we need to understand the why," she said.
Australian Associated Press