PHOENIX - A NINE-YEAR-OLD boy's alleged confession to the double murder of his father and another man will not be used against him, prosecutors said in court documents filed on Tuesday.

The boy is accused of killing both men at the family home in the small rural town of St Johns in eastern Arizona on Nov 5. Both men were shot multiple times with a .22 caliber rifle.

However, if the case goes to trial and the boy takes the witness stand, the statements will be used in court, prosecutors said. The boy is not being identified because of his age.

The boy allegedly made statements to a child case worker while in juvenile detention about keeping a record of his spankings and intending to take action when he reached 1,000 spankings.

Because the boy made the statements without being prompted, prosecutors said none of those statements will be allowed at trial.

The boy's attorney said the case, which has made headlines around the world, may not even go to trial. A psychologist hired by the defence ruled the boy incompetent to stand trial because of his age and intelligence.

'One of the doctors has indicated he is not competent,' attorney Benjamin Brewer told AFP. 'Only the judge can rule whether he's competent or not.'

If the boy is deemed unfit for trial within 240 days, which would include a basic understanding of his rights and the legal process, the two homicide charges could be dismissed.

'If he can't be restored, it's pretty much a done deal,' Ms Brewer said.

A state expert has yet to rule on the boy's competency to stand trial. -- AFP