Suspected cannibal killer Austin Harrouff remains in a critical condition with organ failure after he deliberately ingested caustic chemicals in the garage where the killings took place, US doctors and investigators believe. The 19-year-old college student is suspected of stabbing to death two strangers then chewing the face and abdomen of one of the victims,

Harrouff's health had been something of a mystery until the latest revelations.

He has been in a stable but critical condition in a Florida hospital ever since police said they dragged him off the body of one of his alleged victims as he "grunted" like an animal and bit the dead man's face.

The Florida State University college sophomore is under guard at a Palm Beach hospital, sedated and on a breathing tube, though he was not shot by police, who used a stun gun to subdue him, said Martin County Sheriff Bill Snyder.

Harrouff's liver is malfunctioning, his lungs are filling with fluid and he has bleeding of the oesophagus, his father, Wade Harrouff, told the local CBS station. He may not survive.

He said his son, who is in restraints, sometimes wakes momentarily before going into convulsions. Doctors are planning to do an MRI scan, the Palm Beach Post reported.

The ravages to his body have led doctors to conclude that he ingested poison, according to police.

"It was the hospital's opinion that he drank something," Snyder said. "He had symptoms consistent with having consumed a caustic, poisonous substance."

Investigators believe he may have swallowed dangerously caustic chemicals he found in the Tequesta, Florida, garage where John Stevens and his wife, Michelle Mishcon Stevens, were killed. "It's a typical garage, so there were solvents," said Snyder. "There were things he could have consumed."

Harrouff also nearly cut his own thumb off at some point with a knife he used in the attack, said Snyder.

Harrouff's father said his son's gym shorts were found on the sidewalk some distance from the garage where he had apparently stripped them off before the attack.

Harrouff doesn't believe his son was using drugs but believes he suffered some kind of psychotic break before the attack and began to hurt himself blocks away, as evidenced by a trail of blood drops pointed out to him by a news crew on the sidewalk that led to the garage.

Wade Harrouff said his son showed him his pocket knife the day of the attack, and said odd things like "I will protect you dad" and "they're trying to kill you." Austin "was not mentally well that day. He was making suicidal threats," before dinner, which "concerned" his parents, the father said.

Harrouff had never before been in trouble with the law until police encountered him in the Stevens' garage. But his mother, Mina Harrouff, also said that her son had been acting strangely for days and talking about having "super human" powers.

Harrouff also made a series of videos recently, including some talking about giving up steroid use, and noted in a caption on his YouTube page that he has a "normal side and a psycho side."

Investigators are still reeling over the level of violence in the Stevens' garage, and have not yet determined what drove Harrouff to attack, said Snyder.

When he was arrested he challenged police to test him for drugs, vowing they wouldn't find any, according to officials.

Harrouff has tested negative for most street drugs such as heroin, methamphetamine and marijuana, but results are still pending for synthetic drugs flakka and bath salts, which can trigger extremely bizarre and violent behaviour in users. Apparent cannibalism and stripping off clothes can be signs of flakka use, Synder has said.

The final toxicology reports will provide "a big piece of the unknown," Snyder said.

Police have not yet been able to talk to Harrouff because he is so seriously ill.

If he improves, officials plan to charge Harrouff with two counts of first-degree murder, as well as a count of attempted murder for an alleged attack on a man who came to his neighbours' aid.