WARNING: Disturbing content:

A prosecution psychiatrist says a Florida college student was legally insane when he fatally attacked a couple outside their home, chewing off part of the man's face, court documents filed this week show.

Dr Gregory C Landrum's finding bolsters the case of Austin Harrouff's attorneys, who are planning to argue the 23-year-old should be found not guilty by reason of insanity at his murder trial, which had been scheduled for May but was postponed indefinitely Thursday.

He faces a life sentence if convicted of murdering John Stevens and Michelle Mishcon Stevens in August 2016.

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Landrum outlined Harrouff's declining mental state before the killings, including God and demons talking to him, increasing paranoia and other hallucinations. He noted that Harrouff is being treated for schizophrenia while jailed.

As a result, Landrum concluded, "Harrouff was unable to distinguish right from wrong" when he killed the Stevenses — the legal standard in Florida for being found not guilty by reason of insanity.

If Harrouff is found not guilty by reason of insanity, that does not mean he would go free.

He would be committed to a state mental hospital and his attorneys have previously conceded it is unlikely that he would ever be released.

Harrouff "has a mental illness and, because of the illness, is manifestly dangerous to himself and others," Landrum wrote.

Defense attorney Nellie L. King welcomed Landrum's finding, while understanding "it may provide little comfort to the victims' families."

"However, mental illness is very real and can lead to unintentional, yet tragic, outcomes," King said in a statement.

The defense examiner also found him legally insane. At a pretrial hearing on Thursday (US time), prosecutors asked Circuit Judge Sherwood Bauer for permission to have another psychiatrist examine Harrouff.

Bauer said he would consider it, but warned prosecutors that if this doctor agrees with the first two their case would seem doomed.

Evan Fetterman, an attorney representing Michelle Stevens' family in a lawsuit against Harrouff, said in a statement Thursday that Landrum's "conclusions are inconsistent with the facts" because Harrouff abused drugs in the months preceding the attack and was drunk during it.

That usage caused any insanity Harrouff may have experienced and should not be legally considered, he said.

When Martin County sheriff's deputies arrived at the Stevenses' home north of Palm Beach on the night of August 15, 2016, they found a horrific scene, court documents, recordings and photos show.

Michelle Stevens, 53, lay mangled and dead in the garage and Harrouff, then a muscular exercise science major at Florida State University, was attacking and biting her 59-year-old husband on the driveway.

Harrouff is alleged to have also wounded a neighbour who tried to save the couple.

One deputy ordered Harrouff off at gunpoint while another used an electric stun gun on him, but he wouldn't let go.

Deputies say they didn't shoot Harrouff because they feared hitting John Stevens.

Finally, a deputy with a dog arrived and its bites enabled deputies to subdue Harrouff, who had no previous arrest record.

He told deputies, "Help me, I ate something bad" and then admitted it was "humans" as he spit out a piece of flesh, court documents show. He begged deputies to kill him after they pulled him off Stevens, according to the records.

"Shoot me now; I deserve to die," Harrouff said.

Harrouff has a history of alcohol and drug abuse, including cocaine, methamphetamine and other stimulants and hallucinogens like mushrooms, court records show.

Blood tests found alcohol and marijuana in his system in the hours after the killings.

Landrum wrote that Harrouff in an October interview told him that in the weeks leading up to the killings he had begun to feel "go go go" and experienced grandiosity during the day and paranoia at night.

Demons and God were talking to him. He said it felt like he had Jesus' "special abilities" while working at his job as a dental assistant and that he blessed the dental instruments by pouring water on them.

Harrouff told Landrum that on the day before the killings he felt that "dog spirits" had become part of him and he had new strength and agility.

On the evening of the killings, his mother found him at her home drinking cooking oil mixed with Parmesan cheese — Michelle Stevens' family alleges the concoction was spiked with hallucinogenic mushrooms.

About 45 minutes before the attack, Harrouff argued with his father and stormed out of a restaurant 3.2km from the Stevenses' home.

Harrouff later told the "Dr. Phil" television show that he was fleeing a demon he called "Daniel."

He told Landrum that he recalls having a machete in his hand and stabbing a woman — "it was like she was covered in darkness."

He recalled seeing a man who was "glowing white" and began stabbing and biting him.

He remembers drinking a liquid, calling out to God to save him and feeling he was a dog before blacking out.

Harrouff was hospitalised for nearly two months recovering from injuries caused by drinking a caustic liquid found in the couple's garage.

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
•
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202

- AP