New information has been revealed giving insight into what may have killed pop–star Prince.

A source close to the investigation into his death by overdose says pills seized from inside his Paisley Park compound were labeled as hydrocodone, but actually contained fentanyl – the drug that killed him.

The report, from the Minneapolis StarTribune, says investigators are working under the theory that the singer did not know the pills contained fentanyl.

Four months after Prince's death, questions persist. Likely he became high-profile casualty of new national crisis https://t.co/YzpfIf1b9B

— Stephen Montemayor (@smontemayor) August 21, 2016

Autopsy results showed Prince had so much of the drug in his system that no one, let alone a man of his diminutive size, would have survived it.

The report says Prince did not possess a prescription for fentanyl, a synthetic opioid that is around 100 times more powerful than morphine.

Since his death on April 21 this year, investigators have been wrestling with the question of what exactly happened in his final hours.

His death has also sparked debate in the US over how easy it is to get hold of counterfeit pills, with authorities promising to crack down on opioid abuse.

– with agencies.