Donald Day Jr, 58, was arrested in December 2023 by the FBI in Arizona on two counts of “interstate threats” following the December 2022 fatal shootings in Queensland.

The District Court of Arizona on Thursday (AEST) unsealed a new indictment that added three charges against Day of possessing firearms after being convicted of a felony, possessing an unregistered short-barrelled shotgun and threatening to kill five FBI special agents.

The new indictment alleged Day threatened to kill the agents during his arrest and told them it would be safer to instead kill him and dump his body somewhere.

“If I ever get out of these cuffs, I’ll f***ing come for every one of you … I’m not playing with you guys, I’m not making idle threats here,” Day allegedly said.

Day was accused of possessing nine firearms including four handguns and three military-style rifles – one AK-47 and two AR-15s – along with large caches of ammunition despite previously being convicted of larceny in the US state of Washington.

The indictment also alleged that Day possessed a pump-action shotgun with a barrel that had been shortened without the required federal registration.

US prosecutors sought an order that Day forfeit the firearms if he was convicted.

Constables Matthew Arnold and Rachel McCrow were shot in cold blood by Nathaniel, Gareth and Stacey Train after the officers arrived at a Wieambilla property west of Brisbane.

Neighbour Alan Dare was also shot dead before the Trains were killed in a gunfight with specialist police later that night.

Between May 2021 and December 2022, Day, of Heber, Arizona, is accused of repeatedly sending messages about a “Christian end-of-days ideology” known as premillennialism to the Trains.

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.

US prosecutors alleged Day uploaded a video to YouTube four days later in which he said the Trains “have done exactly what they were supposed to do, and that is kill these f***ing devils”.

Day allegedly later threatened to kill the director-general of the World Health Organisation.

Day’s lawyer, Jon Sands, this week again urged the court to drop the first two charges of interstate threats, claiming that prosecutors lacked proof and legal precedents to support the allegations.

“The government cannot explain how the statement ‘it is time to kill these monsters’ could plausibly constitute a serious statement of personal plan,” Mr Sands said.

Mr Sands earlier this month filed a motion to dismiss the charges claiming Day’s statements were protected by his constitutional right to free speech.

Queensland Police and the FBI have said they are still investigating the alleged contact between the Trains and Day, who has been denied bail.