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.
There is no indication that Prince took his own life, Carver County Sheriff Jim Olson said.
Sheriff Olson added that there were no obvious signs of trauma on the singer's body.
However, police are investigating whether the singer's death involved foul play.
Speaking at a press conference at Carver County Courthouse in Chaska, Minnesota, Sheriff Olson refused to comment on reports that Prince had consumed a powerful painkiller before his death.
He said Prince was last seen at 8pm the night before his death when he was dropped off at his Paisley Park estate.
The singer had been alone at the estate and staff member had been unable to reach him, Sheriff Olson added.
A spokeswoman for the Midwest Medical Examiner's Office said it would not release information from the post-mortem examination until "all results are obtained", and added that the findings of a toxicology scan "could likely take weeks".
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.