A 19-year-old student with no criminal record fatally stabbed a couple at random in their garage, wounded their neighbour and was biting the dead man's face when deputies finally subdued him, authorities in Florida said.
Florida State University student Austin Harrouff may have been on hallucinogenic drugs when he attacked Michelle Mishcon, 53, and John Stevens, 59, at their Tequesta home late on Monday, Martin County Sheriff William Snyder told reporters.
He said Harrouff had been eating dinner with his family at a restaurant a short distance away when he stormed off, apparently agitated about slow service.
His parents were so worried by his behaviour 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," Mr 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'."
Mr Snyder said Harrouff stormed out of the nearby Duffy's, a popular sports bar and restaurant, at about 8.30pm local time.
He said the would-be rescuer called emergency services at 9.20pm to report the attack, which apparently began in the garage where the couple liked to sit at night.
Harrouff is muscular - he wrestled and played football at Suncoast Community High School in nearby Riviera Beach.
When the first deputy arrived minutes later, she used her Taser on Harrouff but it did not faze him, he said.
She tried pulling him off Mr Stevens' body but could not.
Other deputies arrived shortly after along with a dog and it took all of them to subdue Harrouff.
Mr Snyder said they did not shoot, fearing their bullets would strike the victim.
"The suspect in this case was abnormally strong," Mr 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.
Mr Stevens died in the driveway. Ms Mishcon was found dead in the garage.
Mr Snyder said the neighbour who tried to intervene was stabbed and is in hospital but is expected to survive.
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.