Aaron Hernandez's autopsy shows the ex-NFL player had traces of synthetic marijuana in his system at the time of his death, this according to reports ... but the medical examiner is calling it a "rumor."

Newsweek is reporting Hernandez's body tested positive for K2 during a kidney fluid screen conducted during the autopsy. 

The outlet says a full-scale search and raid of the prison was carried out in the wake of the discovery -- with law enforcement officials on the hunt for contraband and clues to how the K2 drug got into the prison in the first place. 

It sounds like a scene out of "Orange is the New Black" -- only not as fun.

Some officials believe K2 can be smuggled into prison through the mail -- since you can dip corners of paper into the liquid for the drug user to consume at a later time.

But we reached out to the Massachusetts Medical Examiner's office and we were told, "The toxicology is not yet complete so I am guessing this is a rumor."

The rep told us there is no timeline for when the results will be in.