Pop music icon Prince died of an accidental overdose of a fentanyl which is a synthetic opoid said to be 50 times more potent than heroin. The Midwest Medical Examiner’s office in Minneapolis said on Thursday that the 57-year-old musician administerd the drug himself, although the date he took it is unknown. He was found dead April 21 at his Minneapolis-area Paisley Park compound.

A law enforcement official close to the case, speaking on condition of anonymity because he was not authorized to speak to the media, tended to confirm suspicions that opioids played a role in the musician’s death. After Prince died, one 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.

Less than one week before his death, Prince’s plane made an emergency stop in Moline, Ill. for medical treatment as he was returning from an Atlanta concert. The official said the singer was found unconscious on the plane and that first responders gave him a shot of Narcan, an antidote used in suspected opioid overdoses.

The names of two doctors have arisen in the death investigation being conducted by the Carver County Sheriff’s office, the U.S. Attorney’s office in Minnesota, and the Drug Enforcement Administration. Dr. Michael Todd Schulenberg, a family practitioner, reportedly treated Prince in the weeks prior to his death and told investigators he had prescribed medications for the singer. Schulenberg said he saw Prince on April 7 and again on April 20. Dr. Howard Kornfeld, a Bay Area addiction specialist, was reportedly asked by Prince’s representatives on April 20 to assist the singer. Kornfeld sent his son, Andrew, on a redeye flight that evening, and the younger Kornfeld was among the people who found Prince unresponsive in an elevator the next morning, according to Kornfeld’s attorney, William Mauzy. The younger Kornfeld is not a physician and carried with him buprenorphine, a medication that can be used to treat opioid addiction by easing cravings and withdrawal symptoms. Mauzy told investigators that Kornfeld intended to give the medication to a nearby doctor who had cleared his schedule to see Prince on April 21.

Musician Sheila E, a longtime friend and collaborator with Prince, told the AP that the singer 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.