Prescription drugs were found in Prince's possession when he died, according to US reports.
Officials told NBC News the painkillers were discovered in his Minneapolis home - but it is as yet unclear if they played any part in his death.
The pop star died aged 57 on April 21 after being found collapsed in a lift at his Paisley Park estate.
Pop superstar Prince had no known will, his sister has said.
Tyka Nelson has filed paperwork to be appointed executor in state court in Minnesota.
The 57-year-old superstar was found dead in an elevator at his Paisley Park Studios compound in Minneapolis on Thursday. No cause of death has yet been released.
Ms Nelson, who is his only surviving full sibling, said in the documents that an emergency exists because immediate action is necessary to manage Prince's business interests.
He is believed to have left around $27 million (£18.5 million) in property and a spike in sales of his music since his death is likely to boost the wealth of his estate.
It is not known exactly how much Prince was worth when he died.
The painkillers were reportedly found in the singer's possession when he died - but it is unclear if they played any part in his death.
Mourners celebrate pop icon at private memorial at Paisley Park estate.
Star's ex-manager voiced concerns over whether whoever inherits the star's fortune, and valuable catalogue of works, will know what to do.