A college student, who officials say killed a southern Florida couple and was eating the face of one of the victims when he was Tasered by police, may not survive according to law enforcement authorities.

Florida State University student Austin Harrouff, 19, was arrested when police found him in the middle of the scene. He suffered bruises, police dog bites and cuts, and stun-gun injuries during his altercation with deputies.

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Harrouff is being held under guard at a hospital at a Palm Beach County hospital, where his condition is worsening and he may not live, said Martin County Sheriff Bill Snyder at a press conference, reports ABC News.

Snyder suspects Harrouff may have been intoxicated on the powerful, trans-formative "zombie drug" known as flakka that has been previously linked to extremely bizarre behaviour. Tests are pending, reports the Palm Beach Post.

Harrouff was a stand-out student and athlete who has never before been in trouble with the law, when he suddenly stormed out of a restaurant where he was eating with his family on Monday night (15 August) in the town of Tequesta.

He ended up nearby, outside a stranger's home, where police say Harrouff used a switchblade he was known to carry to stab to death Michelle Mishcon, 53, and her husband John Stevens, 59, and injured a neighbour who came to their aide.

The couple happened to be sitting in a remodelled garage and had the door open when Harrouff walked up and went after them in what officials called a "completely unprovoked attack."

When police arrived, they found Harrouff "grunting and growling" and "making animal noises" and tearing chunks out of Stevens' face with his teeth on the driveway.

Police used a stun gun and police dog on Harrouff but were initially unable to pull him off the body until other officers arrived on the scene. Officers didn't fire on Harrouff because they didn't want to harm Stevens who they weren't certain was dead.

Harrouff was "abnormally strong" during the confrontation with deputies, said Snyder, who added that obviously there was an "enormous amount of violence" during the incident. He said investigators still have no motive.

"I know what you're thinking, everyone is trying to unwrap this in your head," said Snyder at the press conference. "We're all in the same boat, we're trying to put this together."

Tests on Harrouff for cocaine, opiates, methamphetamine and marijuana were all negative, Snyder said at a press conference.

But tests are still pending for synthetic drugs such as flakka known to cause incredibly strange outbursts.

Harrouff removed some of his clothes during the attack, and was continuing to make "animal-like noises" at the hospital, which, along with the biting of human flesh and his super human strength, are all characteristic of flakka use, noted Snyder. But his body temperature was not elevated, which is also a sign of flakka intoxication.

Flakka has been linked to extremely bizarre behaviour and users have been known to strip naked, deliberately injure themselves and try to have sex with trees. It can be snorted, injected, smoked or swallowed.

Authorities suspected flakka or "bath salts" use when Miami resident Rudy Eugene bit chunks out of a homeless man's face in 2012, but tests only found marijuana in his system. Eugene was shot dead by police during the attack that left the homeless man permanently disfigured.