August 16, 2016 4:36 PM EDT

A 19-year-old college student in Florida grunted and growled as he allegedly attacked a married couple, killing them outside their home in an “inexplicable” assault that may have been sparked by drug use, authorities said.

Austin Harrouff was hovering over one of his victims, John Stevens, and biting the 59-year-old’s face Monday night before deputies arrived and subdued him, Martin County Sheriff William Snyder said Tuesday.

Harrouff, a Florida State University student with good grades and no criminal record, was “exhibiting abnormal levels of strength” and did not respond to being stunned by a deputy’s Taser, being bitten by a police dog or tackled by at least three law enforcement officers. “Nothing was working. They were using all the force they were able to muster physically,” Snyder told reporters at a news conference.

Stevens died on his driveway while his 53-year-old wife Michelle Mishcon was found dead inside the garage. Both were stabbed multiple times and sustained “massive trauma” to their bodies. “There was an enormous amount of violence inside that garage,” Snyder said.

The victims did not know Harrouff, according to the sheriff, who added that the attack was “completely unprovoked and random.”

“It’s inexplicable,” Snyder said. “One of the first things we try to do at a crime scene is try to understand the motive of the offender, because it is the motive of the offender that gets us going in the right direction. In this case, we can’t establish a motive. It’s ‘I don’t know.'”

Authorities could not yet confirm Tuesday that hallucinogen drugs played a role in the double murder, but the suspect exhibited some significant signs of use of either bath salts or the designer drug called flakka, which made headlines early last year.

“I’m not going to speculate, except to say that we know in our business that people on flakka or bath salts will do this type of behavior, where they attack the victim and do the biting and actually remove pieces of flesh in the bites,” Snyder said.

Harrouff was shirtless and was “making animal-like sounds” when he arrived at the hospital. His parents told authorities that they were eating dinner with him at a restaurant before the attack but that their son stormed off after he suddenly became “clearly agitated,” according to Snyder.

He tested negative for use of cocaine, methamphetamines, marijuana and opioids, but more blood work has been ordered, authorities said.

Harrouff’s health has deteriorated since being sedated at a hospital, and he is now in life-threatening condition, Snyder said.

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