A law-enforcement official says tests show Prince died of an opioid overdose.

The 57-year-old singer was found dead April 21 at his Minneapolis-area estate and the findings confirm suspicions that opioids played a role in the musician's death.

The official, who is close to the investigation, spoke to the Associated Press on condition of anonymity because he was not authorised to speak to the media.

Investigators have been reviewing whether Prince died of an overdose and whether a doctor was prescribing him drugs in the weeks before his death.