Prince died of an opioid overdose, a law enforcement official has said.
The 57-year-old singer was found dead at his Minneapolis-area estate on April 21.
The findings confirm suspicions that opioids played a role in the musician's death. After he died, a law enforcement official told the AP that investigators were examining whether an overdose was to blame and whether a doctor had prescribed him drugs in the preceding weeks.
Prince's death came less than a week after his plane made an emergency stop in Moline, Illinois, for medical treatment as he was returning from an Atlanta concert. The official said Prince was found unconscious on the plane and that first responders gave him a shot of Narcan, an antidote used in suspected opioid overdoses.
At least two doctors' names have come up in the death investigation being conducted by the Carver County Sheriff's Office, the US Attorney's Office in Minnesota and the Drug Enforcement Administration.
Dr Michael Todd Schulenberg, a family practitioner, treated Prince twice in the weeks before his death and told investigators he prescribed medications for the singer. The medications were not specified in a search warrant for the Minnesota hospital that employed Schulenberg at the time.
Schulenberg saw Prince on April 7 and April 20 - the day before his death - according to the warrant. Schulenberg's attorney has declined to comment on the case.
Dr Howard Kornfeld, a California addiction specialist, was asked by Prince's representatives on April 20 to help the singer.
Kornfeld sent his son Andrew on a redeye flight that night, and Andrew was among the people who found Prince's unresponsive body the next morning, according to Kornfeld's attorney, William Mauzy.
The younger Kornfeld, who is not a doctor, was carrying buprenorphine, a medication that can be used to treat opioid addiction by easing cravings and withdrawal symptoms, Mauzy said, explaining that Andrew Kornfeld intended to give the medication to a Minnesota doctor who had cleared his schedule to see Prince on April 21. Mauzy has refused to identify that doctor. Schulenberg is not authorised to prescribe buprenorphine.
Prince's death came two weeks after he cancelled concerts in Atlanta, saying he wasn't feeling well. He played a pair of make-up shows on April 14 in that city, and then came the emergency landing in Moline. Prince was scheduled to perform two shows in St Louis but cancelled them shortly before his death.
The superstar had a reputation for clean living, and some friends said they never saw any sign of drug use. But long-time friend and collaborator Sheila E. has told the AP that Prince had physical issues from performing, citing hip and knee problems that she said came from years of jumping off risers and stage speakers in heels.