Tequesta - A 19-year-old Florida State University student charged with fatally stabbing a couple outside their home and then biting the dead man's face remained under surveillance early on Wednesday with guards nearby at a West Palm Beach hospital.Martin County Sheriff William Snyder said Austin Harrouff was incoherent and making "animal-like noises" when he was taken to St. Mary's Medical Center after the Monday night attack in a quiet neighbourhood north of Jupiter.On Tuesday, Snyder told reporters that Harrouff's condition was "life-threatening." West Palm Beach television station WPTV reported on Wednesday that Harrouff is sedated and on a ventilator with guards stationed nearby.Snyder said Harrouff stormed out of a restaurant where he was having dinner with his family on Monday night, apparently agitated over slow service.A short time later, Michelle Mishcon, 53, and John Stevens III, 59, were attacked outside their home. Neighbours said they had a habit of sitting in their garage, where they often kept the door open while they watched television.Neighbour stabbed in the neckNeighbour Jeff Fisher, 47, tried to help but was stabbed during the attack. He was breathing heavily on the 911 call, which was released on Tuesday. He told the operator that the assailant had attacked a woman and then him when he tried to intervene. Fisher was bleeding profusely and taken to a hospital. His father, Steve Fisher, told WPTV that his son was stabbed once in the neck, three times in the back and once in the side. He's expected to survive.When a deputy arrived at the couples' home, she used her Taser on Harrouff, but it didn't faze him, Snyder said. She tried pulling him off Stevens' body, but couldn't.Other deputies arrived shortly along with a dog and it took all of them to subdue Harrouff. Snyder said they didn't shoot, fearing their bullets would strike the victim.Drug tests negative"The suspect in this case was abnormally strong," Snyder said, making him think Harrouff was on drugs. He said hospital blood tests showed no signs of methamphetamine, cocaine, heroin or other common drugs; it will take longer to test for less common hallucinogenic drugs such as flakka or bath salts.Dr Wade Harrouff, a dentist, declined to comment about his son's arrest.Attorney Michelle Suskauer told WPTV she'll be representing Austin Harrouff if he survives. She told the television station that his parents have not been able to see him, but she's continuing to request permission for them to be by his side.She said he is in serious condition in the hospital's intensive care unit."Obviously, we don't know what predicated these acts. But certainly, there are mental health issues that are going to have to be investigated here. We don't know if anything is going to show up on toxicology or not," Suskauer said.