CHICAGO - A Minnesota medical examiner says Prince died of an accidental fentanyl overdose.

The report from the Midwest Medical Examiner's Office was issued Thursday, more than a month after the music superstar was found dead at age 57 at his Paisley Park mansion.

The single-page report said Prince "self-administered fentanyl," referring to a synthetic opioid many times more potent than heroin. The report was signed by Quinn Strobl, the office's chief medical examiner.

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. He was found dead April 21 in his suburban Minneapolis home. The Associated Press and other media reported, based on anonymous sources, that Prince was found unconscious on the plane, and 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 U.S. Attorney's Office in Minnesota and the Drug Enforcement Administration; feds officially joined the Carver County Sheriff's investigation in the beginning of May.

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A search warrant in May revealed that Dr. Michael Todd Schulenberg, 46, treated Prince on April 7 and April 20 and prescribed him medications, though it didn't specify the drugs or say whether Prince took them.

That month, a law enforcement official told the Associated Press that investigators were looking into whether Prince died from an overdose and whether a doctor was prescribing him drugs in the weeks before his death.

Investigators interviewed Schulenberg and searched a suburban Minneapolis hospital where he worked, according to the warrant. 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.

"Andrew's purpose for being there was to describe the recovery without walls program to familiarize Prince with that. So Prince could go there for pain management and addiction issues," said Mauzy to CBS News' Jamie Yuccas.

Mauzy has refused to identify that doctor. Schulenberg is not authorized to prescribe buprenorphine.

Prince's death came two weeks after he canceled concerts in Atlanta, saying he wasn't feeling well. He played a pair of makeup shows April 14 in that city, and then came the emergency landing in Moline. He was scheduled to perform two shows in St. Louis but canceled 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 longtime 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.