A father who beheaded his disabled son has been found not guilty by reason of insanity by a US court, after experts concluded he believed his son was actually a CPR dummy.

In a rare ruling on Friday, State District Judge John LeBlanc said Jeremiah Wright of Louisiana will not face trial for capital murder after expert testimony found he was psychotic and delusional at the time he committed the act.

Experts on mental illness said Wright believed his son was no longer real, and had instead been replaced with a CPR dummy.

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Wright has been in custody since August 2011, when his seven-year-old son Jori Lirette was killed at the home he shared with the boy's mother.

The boy's head was found in the driveway of the home and his other body parts were found in nearby trash bags.

Jori's disability required him to need around-the-clock care, including being fed through a tube.

Wright has believed from the start that he had dismantled a CPR dummy that had taken his son’s place, his lawyer Kerry Cuccia said.

“That’s what he told the police in his initial statement and what he has told almost every doctor along the way.”

Both Dr. Sarah Deland, an assistant professor of psychiatry and forensic neuropsychiatry at Tulane University, and Dr. George Seiden, a forensic psychiatrist from Shreveport, brought in by prosecutors, testified that even when taking psychiatric medicines, Wright never stopped believing that a robot or CPR dummy had replaced his son, Cuccia said.

Wright has now been committed to the the East Louisiana Mental Health System in Jackson, where he has been held since Joris was killed.

Prosecutor Camille 'Cam' Morvant said: "It was the only ruling, really, that the court could make.

"There was indisputable testimony from experts that Mr Wright was psychotic and delusional and suffered from a major mental disorder at the time of the crime."

Additional reporting by Associated Press