Prince died of an accidental overdose of the powerful painkiller fentanyl, autopsy results released Thursday show.

The 57-year-old singer was found dead April 21 at his Minneapolis-area estate.

According to a one-page report released by the Midwest Medical Examiner's Office, Prince administered the drug himself, but the date he took it was unknown. The office said it has completed its death investigation and had no further comment.

Fentanyl is a synthetic opioid, 50 times more potent than heroin, that's partly responsible for a recent surge in overdose deaths in some parts of the country. It also has legitimate medical uses. More than 700 fentanyl-related overdose deaths were reported to the Drug Enforcement Administration in late 2013 and 2014.

The findings confirm suspicions that opioids played a role in the musician's death. After he died, authorities began reviewing whether an overdose was to blame and whether he had been prescribed 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 AP and other media reported, based on anonymous sources, that the superstar was found unconscious on the plane, and first responders gave him a shot of Narcan, an antidote used in suspected opioid overdoses.

Doctors, medications investigated

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.

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 April 7 and April 20 — the day before his death — according to the warrant. Schulenberg's attorney has declined to comment on the case.

Flowers hang from the fence outside Paisley Park, Prince's home and studio complex where was found dead. (Cameron MacIntosh/CBC)

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 authorized to prescribe buprenorphine.

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Prince's death came two weeks after he cancelled 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. Prince 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.

Julya Baer, 30, cries at a vigil to celebrate the life and music of Prince in Los Angeles after the musician died at age 57. (Lucy Nicholson/Reuters)

Some key events in the last two weeks of Prince's life:

April 7 — Dr. Michael Schulenberg, a Minnesota primary care physician, sees Prince, according to the search warrant. Also, two Prince concerts in Atlanta are postponed. The artist said at the time he had fallen ill with the flu.

April 14 — Prince performs makeup concerts in Atlanta, apologizing to fans. He jokes about having been "under the weather," giving a slight smile. His voice seems a bit weak at times while speaking, but sounds fine when singing during his 80-minute show.

April 15 — Prince falls ill on a flight home from Atlanta, and the plane makes an emergency stop in Moline, Ill. A law enforcement official who spoke to the AP on condition of anonymity because he was not authorized to speak to the media said that Prince was found unconscious on the plane and that first responders gave him a shot of Narcan, an antidote used to reverse suspected opioid overdoses.

April 16 — Prince hosts a dance party at his Paisley Park complex and makes a brief appearance, showing off a new purple piano. "Wait a few days before you waste any prayers," he tells fans.

April 20 — Prince is seen by Schulenberg again, according to the warrant. At some point, Schulenberg prescribed medications to Prince and ordered tests, according to the warrant, which does not specify what medications were prescribed or whether Prince took them.

April 20 — Dr. Howard Kornfeld, a California addiction specialist, is asked by Prince representatives to help the star, according to Kornfeld attorney William Mauzy. Kornfeld sends his son, a non-physician, on a red-eye flight to Minnesota, carrying a drug used to treat opiate addiction.

April 21 — Andrew Kornfeld and others find Prince unresponsive in an elevator at Paisley Park. Schulenberg arrived "on the death scene" at some point, according to the warrant. He tells a detective he was there to drop off test results, and that he had prescribed medications that were to be filled at a Walgreen's pharmacy.