The two were killed outside their home in Tequesta by 19-year-old Austin Harrouff, according to investigators.
Snyder said tests for substances in Harrouff's system may help explain why the student with no history of criminal behavior became extraordinarily violent.
Harrouff's concerned mother phoned police after her son angrily left a restaurant, anxious about his unusual behaviour and comments about being "a superhero". Authorities released surveillance video on Friday that showed the teen leaving the restaurant.
Mina Harrouff told the emergency operator her son was carrying a pocket knife and wearing "a red hat. a Make America Great Again hat".
Martin County Sheriff William Snyder said deputies found Harrouff on top of Stevens, biting and removing pieces of the victim's face with his teeth.
After a horrific attack in Florida, the family of the suspect is offering its condolences.
The 19-year-old Harrouff was hospitalized following the attack, and authorities were not immediately sure if he would survive.
On Friday, Harrouff's family released a statement that read in part: "The Harrouff family expresses their deepest sympathy for the pain and suffering caused by Austin Harrouff".
We love our son and know that he is not the person some are making him to be for their own purposes.
John Joseph Stevens III, 59, and Michelle Karen Mishcon, 53, were killed in the attack. That means he faces the death penalty or life without parole if convicted.
A university student who was found biting a man's face after allegedly killing him and his wife in the USA claimed he had "superpowers". The Associated Press reported that Snyder earlier suggested that he is not in a hurry to charge Harrouff because his family or insurance will have to pay the hospital bill until he is charged.
Stevens' son John Stevens IV tells the Sun Sentinel he wants prosecutors to seek the death penalty.
Harrouff is conscious but heavily sedated and under armed guard in a hospital, according to the sheriff's office. His tests came back negative for common street drugs and methamphetamines. It takes longer to test for less common hallucinogenic drugs such as flakka or bath salts.
He said: "I just think that this kid had a steroids dependency and had a bad, bad roid rage that night".
A Florida university student, who allegedly killed two strangers and tried to bite the face of one of the victims, will be charged with first-degree murder.
Fisher told deputies that Mishcon and Stevens were still alive when he got there on Monday night, before being forced to flee after he was stabbed. They said they want him to face execution.
Fisher suffered multiple stab wounds when he tried to intervene before calling 911.
Harrouff, who had surgery on his hand Wednesday at St. Mary's Medical Center in West Palm Beach, remained at the hospital in stable condition, said sheriff's spokeswoman Trisha Kukuvka.