Donald Day Jr, 58, has been arrested by the FBI in Arizona after a US federal grand jury indicted him on two counts of "interstate threats" following the December 2022 fatal shootings in Queensland.
One of the indictments is connected to what was described as Australia's first religiously-motivated domestic terror 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 west of Brisbane on December 12 last year.
Neighbour Alan Dare was also shot dead before the Trains were killed in a gunfight with specialist police later that night.
Constables Randall Kirk and Keely Brough escaped the attack which investigators said was motivated by extremist Christian beliefs.
Charges have been laid over the attack that killed Constables Matthew Arnold and Rachel McCrow. (HANDOUT/QUEENSLAND POLICE)
A joint Queensland Police-FBI investigation is ongoing, with evidence seized from a remote property near Heber-Overgaard, Arizona where Day was arrested last week.
The second indictment against Day relates to an alleged online threat to kill the World Health Organization director general earlier this year.
Each count carries a maximum US jail term of five years.
Queensland Police Commissioner Katarina Carroll was hopeful more charges would be laid in the future.
"People need to be held to account for inciting that," she said of the 2022 attack on Thursday.
"That is deplorable behaviour, particularly what he was inciting people for - motivating against police, against authorities - which is completely unacceptable."
The indictment connected to the shootings relates to a December 2022 YouTube video Day allegedly posted after the Wieambilla attack, "threatening any law enforcement official who came to his residence", a US Attorney's Office of Arizona statement said.
In an unsealed indictment, it is alleged Day posted a YouTube video on December 16 under the name Geronimo's Bones that referred to Gareth and Stacey Train as "Daniel and Jane" and threatened violence against police.
"My brave brother and sister, a son and a daughter of the Most High have done exactly what they were supposed to do, and that is to kill these f***ing devils," Day allegedly said in the video.
He is accused of later saying: "Like my brother Daniel, like my sister Jane, it is no different for us.
"The devils come for us, they f***ing die. It's just that simple. We are free people, we are owned by no one."
The shooting has been described as a religiously-motivated terrorist attack. (Jono Searle/AAP PHOTOS)
It is alleged Gareth Train began following Day's YouTube account from May 2020.
The men began commenting on each other's videos a year later, police said.
Between May 2021 and December 2022, Day is accused of repeatedly sending messages containing "Christian end-of-days ideology" to the Trains.
The Trains subscribed to a broad Christian fundamentalist belief system known as premillennialism.
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 love to someone called Don.
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