Results of a post mortem may not be made public for another three to four weeks
Detectives are looking into whether Prince was being prescribed drugs by a doctor in the weeks before his death - and whether the star died of an overdose.
A powerful painkiller such as Percocet may have been involved in the 57-year-old's death at his home in Minneapolis, according to anonymous law enforcement officials speaking to the Associated Press news agency.
They also confirmed US media reports that prescription drugs were at the Paisley Park complex where Prince was found dead, but refused to give further details about the type of pills discovered.
One line of enquiry for investigators is whether a doctor was on board Prince's private jet when the singer fell ill in the days before he died.
The plane made an emergency landing in Illinois and first responders gave him a shot of Narcan - an antidote which is often used in suspected opioid overdoses.
Although the investigation into the performer's death is in the early stages, the involvement of a doctor could create parallels with other celebrity deaths.
Conrad Murray, Michael Jackson's doctor, was convicted of involuntary manslaughter after he prescribed the music legend with a powerful anaesthetic which contributed to his death in 2009.
Officials in Minnesota have already performed Prince's post mortem examination, but results may not be made public for another three to four weeks.
The death of the intensely private musician, whose hits included Purple Rain and When Doves Cry, shocked millions of fans around the world and prompted glowing tributes from fellow performers.