After his arrest, Austin Harrouff allegedly challenged police to "test me...you won't find any drugs"

FSU Student Who Allegedly Tried to Eat Man's Face Drank Chemicals in Victim's Garage: Police

Before allegedly fatally stabbing a couple and trying to eat one victim’s face, police say the 19-year-old Florida State University student likely consumed unspecified chemicals he found in the couple’s garage, PEOPLE confirms.

A police spokesperson tells PEOPLE investigators have been unable to interrogate Austin Harrouff, who they believe drank some hazardous household chemicals moments before allegedly attacking 59-year-old John Stevens and his 53-year-old wife, Michelle Mishcon.

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Based on tests performed at St. Mary’s Medical Center in West Palm Beach, where he remains in critical but stable condition, hospital officials suspect Harrouff ingested “something caustic” the night of the killings.

The police spokesperson tells PEOPLE Stevens’ garage contained several solvents and other dangerous solutions, such as paint thinners and chemical cleansers.

Police can’t say with confidence whether Harrouff drank the chemicals to hurt himself.

The spokesperson also tells PEOPLE that, soon after his arrest, Harrouff allegedly challenged the police who’d detained him to test him for drugs, saying: “Test me…you won’t find any drugs.”

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Authorities initially believed Harrouff may have been under the influence of the synthetic drug flakka when he allegedly attacked the couple and tried to eat Stevens’ face.

Harrouff tested negative for common street drugs such as heroin, cocaine, and marijuana, but more extensive testing, which might take up to three weeks, is being performed by the Federal Bureau of Investigation to determine whether Harrouff was under the influence of bath salts or flakka.

Police have yet to release the results of the couple’s autopsies, which showed Mishcon died from blunt force trauma injuries and Stevens died from multiple injuries.

Once he’s released from the hospital, Harrouff will be charged with two counts of first-degree murder as well as attempted first-degree murder for allegedly stabbing the couple’s neighbor, 47-year-old Jeff Fisher, who tried fending off their attacker.

He also faces burglary and resisting arrest charges.

Investigators say they are still trying to determine what allegedly drove Harrouff to commit last week’s rampage.

Harrouff’s parents issued a statement over the weekend, expressing their condolences. “There are no words we can offer to give any real comfort for this tragic loss,” the statement reads. “There are no words we can offer that we can make up for the injury sustained by Jeff Fisher who is doing what we would want all good neighbors to do which was to try to help innocent people from being hurt or killed.”

On Friday, the Martin County Sheriff’s Office released video showing Harrouff walking out of the Duffy’s Restaurant in Jupiter, Florida, the night of the attack.

It is unclear whether Harrouff has retained legal counsel.

In YouTube videos uploaded four days before the attack, Harrouff is seen discussing steroid use. In other clips, Harrouff appears to lift weights and makes references to himself as “The God of Fitness.”