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

Investigators have been reviewing whether Prince died of an overdose and whether a doctor was prescribing him drugs in the weeks before his death.

Legal experts say the finding that Prince died of an accidental overdose of the synthetic opioid fentanyl could make the prospect of criminal charges more likely.

A Chicago-based attorney with no link to the case says the substance, while it has medical applications, is frequently associated with illegal trafficking.

Gal Pissetzky also explains that categorizing the death as accidental indicates only that it was not intentional. It does not preclude charges if the fentanyl was supplied illegally.

The illegal distribution of fentanyl resulting in death carries a mandatory minimum 20 years behind bars in federal court.

The same offense can mean third-degree murder charges in Minnesota and up to 25 years in prison.

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, Illinois. 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.