"It's a tragedy without words to describe," said Martin County Sheriff William Snyder.
"We have two people -- middle-age people -- sitting on their couch enjoying the night and out of nowhere for reasons we may never know, they are attacked by a man with a knife who overcomes them both and kills them," he added. "To say it's sad somehow misses it. I'm not sure I have the words to tell you how I feel."
Authorities arrived at a home near Tequesta on Monday night after receiving a 911 call of a fight across the street.
"As the deputy approached, they observed the suspect biting the male victim in the face," the sheriff's office told CNN.
Deputies attempted to Tase the suspect, identified as Austin Kelly Harrouff, 19. A fight ensued between authorities and Harrouff, who was critically wounded before being taken into custody.
The couple, Michelle Mishcon, 53, and John Stevens III, 59, were stabbed multiple times, police said.
A neighbor who called 911 and came to the aid of the couple was also stabbed but is expected to survive.
The sheriff's office says it doesn't know why the couple was targeted, saying there was "no connection between the victims and the suspect."
Harrouff lived with his mother about three miles away and his father also lived in the immediate neighborhood, according to authorities. He is a student at Florida State University, authorities said. Fall semester is set to start August 29.
Harrouff had been eating dinner at a restaurant with his family when he stormed off, agitated, officials said. His parents were so worried by his behavior that they called police and some of his fraternity brothers, according to a timeline provided by the sheriff's office.
"Standing there last night, looking at that scene, I was forced to think about the futility of the whole thing -- the randomness of it -- as it began to become clear to us that there was no nexus between the victim and the suspect," Snyder said in a press conference.
Harrouff was in critical condition Tuesday and has only provided minimal details to police.
Calls to Harrouff's family were not immediately returned.