Sanders, 18, who starred in the television shows 9-1-1: Lone Star and Fear the Walking Dead, was found dead inside his Los Angeles home on June 16.
Sources with knowledge of the investigation told TMZ that the actor had a history of drug use and confirmed that police located a plastic straw and white powder in the room where Sanders' body was located.
Sanders told a friend that he was using fentanyl in text messages sent on the night before his death, the medical examiner wrote in the autopsy report, according to TMZ.
Sanders didn't respond to calls made by the friend following the text confirming his use of fentanyl, the report states.
The coroner also confirmed that Sanders had no known medical issues but had used several drugs in the past including heroin, cocaine, LSD, mushrooms and Xanax, according to TMZ.
Sanders was reported to be found unresponsive in bed when officers arrived at the scene of his home, where he lived alone and illegal drugs were discovered inside a bathroom.