Pills taken from one of Prince's compounds have been found to contain powerful opioid fentanyl.
Pills recovered from Prince's Paisley Park estate were falsely labelled and actually contained a drug 50 times more potent than heroin, officials have revealed.
A number of tablets discovered throughout the musician's Minneapolis home tested positive for the powerful synthetic opioid fentanyl, an offical speaking on condition of anonymity told news agency AP.
Post-mortem results in June revealed Prince, 57, died of an accidental fentanyl overdose.
:: Prince Died Of Accidental Painkiller Overdose
Records show Prince had no prescription for any controlled substances in the state of Minnesota in the 12 months before he died and it remains unclear how he obtained the drugs.
Fentanyl has been partly responsible for a recent surge in overdose deaths in some parts of the US.
When made into counterfeit pills, users don't always know they're taking fentanyl, increasing the risk of fatal overdose.
The Minneapolis Star Tribune said: "It seems more and more likely that Prince became a casualty of what is being called a new national crisis of deadly counterfeit pills."
:: Painkiller That Killed Prince On The Rise In US
Around a dozen tablets were found in a dressing room in Paisley Park.
The vast majority, however, were found inside aspirin and vitamin bottles tucked in a suitcase and bags, including one that Prince often carried with him.
Some tested positive for not only fentanyl, but also lidocaine and U-4770 - a synthetic drug that is eight times more powerful than morphine.
Around two dozen pills were found to have been falsely labelled as "Watson 385", a stamp used to identity pills containing a mix of acetaminophen and hydrocodone, a weaker form of opioid.
Prince was found unresponsive and alone in an elevator at his complex in Paisley Park, Minnesota, on 21 April.
Emergency services attempted CPR on arrival but could not revive the singer-songwriter.