The singer died of an apparent accidental overdose in April
It is being reported that pills found at Prince's home following his death in April had been mislabelled.
The pills were seized from Prince's Paisley Park estate near Minneapolis after his death.
Investigators - talking to the Minneapolis Star Tribune newspaper - say he may not have even known he was taking the dangerously powerful opioid fentanyl.
The report suggests that the artist did not have a prescription for the powerful drug, which is traditionally used to treat severe pain.
The singer apparently only weighed 8 stone (51kg) when he died in an accidental overdose.
An autopsy report gave the cause of death as "fentanyl toxicity", with the drug self-administrated.
Investigators are reportedly still attempting to find out how Prince ended up with drug, with the Tribune suggesting it appears "more and more likely that Prince became a casualty" of counterfeit pills.
A recent report by the US Drug Enforcement Administration (DEA) found that "hundreds of thousands of counterfeit prescription pills, many containing deadly amounts of fentanyl and fentanyl-related compounds, have made their way into the US drug market".
The report adds that fentanyl "poses a grave threat to law enforcement officials and first responders, as a lethal dose of fentanyl can be accidentally inhaled or absorbed through the skin".