The FBI has arrested a man in Arizona over a religiously-motivated terror attack on a remote Australian property last year that left six dead.
The 58-year-old US citizen faces two charges, one of which relates to the incitement of violence online.
He "repeatedly" sent online messages containing "Christian end of days ideology" to the attackers, police say.
The Wieambilla ambush left two police officers and a neighbour dead, as well as the three shooters involved.
The US man was arrested by FBI agents in Heber Overgaard, north-east of Phoenix, on Friday.
Police say the three shooters - Nathaniel, Stacey and Gareth Train - followed two YouTube accounts created by the man. They added that he continually messaged Gareth and Stacey in the lead up to the attack, which took place in Queensland last December.
There is also evidence that the suspect and Gareth were "commenting directly" on each other's YouTube videos, Queensland Police Assistant Commissioner Cheryl Scanlon told a press conference on Wednesday.
Queensland Police investigators have travelled to the US to assist the FBI and local law enforcement with the investigation.
The deadly attack took place on 12 December after police arrived at a remote inland property owned by Gareth and Stacey Train in Wieambilla - about 270km (168 miles) west of Brisbane.
Four officers had travelled there to check on Nathaniel Train, who had been reported missing to authorities.
As soon as they left their car to approach the house, they were inundated with gunfire.
Two constables - Matthew Arnold, 26, and Rachel McCrow, 29 - were hit immediately, before reportedly being shot again, execution-style.
Another officer was injured but escaped, while the fourth was terrorised by the shooters who lit fires to try and flush her out of hiding.
58-year-old Alan Dare, a neighbour who turned up at the scene to help, was fatally shot too.
The siege involved "many weapons" and continued for hours, before the suspects were shot by specially trained officers, authorities said.
Investigators say the attack was premeditated, and that it involved "advanced planning and preparation against law enforcement".
Camouflaged hideouts, barriers, dirt mounts, guns, knives, CCTV, and mirrors on trees were set up throughout the property.
Queensland police have since labelled the incident a "religiously-motivated terror attack" and allege the Trains subscribed to "a broad Christian fundamentalist belief system known as premillennialism".
Premillennialism is the belief that after a period of destruction and extreme suffering on Earth, Jesus Christ will physically return to bring peace.
It is the first time an extreme Christian ideology has been linked to a terror attack in Australia, authorities say.
The man has been charged under US law and there are currently no plans to extradite him to Australia.
Gun violence is rare in Australia, which has some of the toughest firearms laws in the world.
