TEQUESTA, Fla. » A 19-year-old Florida State University student with no criminal record fatally stabbed a couple at random in their garage, wounded their neighbor and was biting the dead man’s face when deputies finally subdued him, authorities said today.

Martin County Sheriff William Snyder told reporters that Austin Harrouff may have been on hallucinogenic drugs when he attacked Michelle Mishcon, 53, and John Stevens, 59, at their Tequesta home late Monday.

The sheriff said Harrouff joined his family for dinner at a restaurant a short distance away when he stormed off, apparently agitated about slow service. His parents were so worried by his behavior that they called police and some of his fraternity brothers in a futile attempt to find him before the attack.

About 45 minutes later, he apparently targeted the couple at random, the sheriff said.

“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.’”

Snyder said Harrouff stormed out of the nearby Duffy’s, a popular sports bar and restaurant, at about 8:30 p.m. He said the would-be rescuer, identified by family as 47-year-old Jeff Fisher, called 911 at 9:20 p.m. to report the attack, which apparently began in the garage where the couple liked to sit at night.

The son of a dentist, Harrouff is muscular — he wrestled and played football at Suncoast Community High School in nearby Riviera Beach, where rosters listed him at 6-feet-tall and 200 pounds. He’s been a sophomore at FSU, majoring in exercise science.

When the first deputy arrived minutes later, she used her Taser on Harrouff, but it didn’t faze him, he 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.

“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.

Stevens died in the driveway. Mishcon was found dead in the garage.

Stevens owned a lawn service and the couple enjoyed boating, neighbors said.

“John and Michelle were the nicest people,” neighbor Amy Lourie said. She said they would sit in the garage with the door open while watching television and wave and talk to passersby while their Labrador retriever played in the yard. She said they would drive around the neighborhood in their golf cart with the dog sitting with them.

“It was the cutest thing,” Lourie said.

She said it isn’t surprising Fisher would try to rescue them, saying he and the couple were close friends. Fisher’s father, Steve Fisher, told WPTV his son was stabbed five times — once in the neck, three times in the back and once in the side. He is expected to survive.

Dr. Wade Harrouff declined to comment about his son’s arrest. It is unknown if Austin Harrouff has an attorney.

Tequesta is a small, affluent community on the Atlantic coast about 20 miles north of Palm Beach.

Flakka had a brief burst in popularity in late 2014 and 2015, primarily in South Florida, but its usage quickly waned after stories circulated about users’ deaths and mental breakdowns, and a crackdown was carried out in China, where it has been manufactured.