Prosecutor Gary Restaino last week asked for more time for pretrial motions as they had made an undisclosed offer to Day in return for entering a plea, documents filed in the US District Court of Arizona show.

"A plea offer has been extended and the deadline for acceptance is May 17, 2024," Mr Restaino said.

The US Department of Justice states on its website that prosecutors can engage in plea bargaining with defendants if they are willing to plead guilty in court.

"When the government has a strong case, the government may offer the defendant a plea deal to avoid trial and perhaps reduce his exposure to a more lengthy sentence," the department states.

Queensland police in January 2024 started providing confidential evidence to the FBI concerning the killing of the two officers and a Wieambilla resident. 

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

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.

Last week in the US District Court Judge John Tuchi granted an order to extend pretrial motions for Day's trial.

"This Court finds that an extension of the pretrial motion deadline will provide time for plea negotiations and should not interfere with the upcoming trial date," Judge Tuchi stated.

The start date for Day's trial has been set at June 4 but that schedule has previously been extended.