The 57-year-old singer was found dead April 21 at his Minneapolis-area estate.
The official, who is close to the investigation, spoke on condition of anonymity because he was not authorized 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.
TMZ was already reporting that Prince overdosed on Percocet the week before his death and was likely the reason his plane had to make an emergency landing on April 15.
Sources claim that the musician was first prescribed the painkiller when he began experiencing hip problems in 2009.