Prince reportedly had opioid medication on him when he died
A law enforcement official told CNN the pills, often used to treat pain, were found on Prince.
The source also said investigators have brought in the U.S. Drug Enforcement Administration to help with the case.
The musician's private plane made an emergency landing days before his death. His publicist said it was because of a flu, but TMZ reported that sources said Prince had suffered a drug overdose.
Attorney Michael Padden told the Star Tribune that Prince's siblings, Lorna Nelson and Duane Nelson, had discussed with him Prince's problem with Percocet and cocaine over a decade ago. Others close to Prince dispute this, however.
The legendary artist died at the age of 57 in his Paisley Park compound, without a will, leaving lingering questions about what will happen to his $250 million estate.