The negative results mean no foreign substances were found in his system at the time of his death.
Brendan's body was found in a wooded area on the morning of Oct. 13 in Haddon Township, New Jersey, three hours after he was reported missing by his father, 22-year-old David "D.J." Creato, in a 911 call.
The preliminary autopsy results were deemed "inconclusive."
David Creato has been questioned by police, but his attorney Richard Fuschino Jr. says they are still waiting to get more information.
"We don't even have that information from the investigators yet that someone had something to do with this boy's death. We have no answers about what happened to this boy. And after today we have one answer that just really leads to more questions," Fuschino said.
In a statement, the prosecutor's office says they are "continuing to conduct an exhaustive investigation" in Creato's death.
"We have no further comment at this time. We will contact reporters when we have more information to share," the statement read.
Anyone with information is asked to contact the Camden County Prosecutor's Office at (856) 225-8561 or Haddon Township Police at (856) 833-6208.