Donald Day Jr, hailing from Arizona, recently appeared in a U.S. court following the discovery of a large cache of weapons and ammunition at his rural residence, which also included a sniper’s nest.

As part of a plea agreement approved by an Arizona judge, the previously convicted felon admitted to unlawfully possessing firearms and ammunition.

Initially charged with making threats against public figures and FBI agents, these charges were dismissed as part of the plea bargain.

U.S. prosecutors revealed that Day, who is recognized online by the alias “Geronimo’s Bones,” had communicated through social media with Gareth and Stacey Train in Queensland around the time of the fatal Wieambilla ambush.

In 2022, the Trains, together with Nathaniel Train, were responsible for the murders of Queensland police officers Matthew Arnold and Rachel McCrow, as well as a neighbor, Alan Dare, in Wieambilla.

The Trains met their end in a confrontation with police during a subsequent siege at their rural property.

Investigators established direct links between Day with Gareth and Stacey Train through posts from “Geronimo’s Bones”.

Day described Queensland police as “malignant, malformed and malevolent”, should be shown “absolutely no quarter”, telling the Trains he wanted to be at Wieambilla in person.

Court documents detailed how Gareth and Stacey Train had posted an apocalyptic video on YouTube after the shootings, saying authorities “came to kill us and we killed them”.’

“If you don’t defend yourself against these devils and demons, you’re a coward … we’ll see you at home, Don. Love you,” the Trains said.

Day was later found with a cache of nine high-powered firearms and hundreds of rounds of ammunition at a rural property in Heber, Arizona, that was equipped with a gun range and sniper hide, court documents showed.

“The firearms and ammunition were kept in the mobile home I shared with SS, in a room we called the ‘gun room’,” Day admitted in the plea deal.

Day said he regularly accessed both the gun room and the weapons and also instructed others on how to use the firearms correctly.

According to court documents, Day had been prohibited from possessing weapons and firearms because of his violent criminal history.

Day faces a maximum penalty of up to 15 years in prison or a fine of $US250,000 ($A381,500) after pleading guilty to the single charge.

However, the plea deal for Day – who has served two years in custody – stipulates he will be sentenced under the low end of the sentencing guidelines.

Day is set to be sentenced on January 8.