Prince died of an accidental drug overdose, according to the medical examiner who performed the late superstar's autopsy.

Prince, who was found dead at his Minnesota estate on April 21, was killed by an overdose of the painkiller fentanyl, an opioid that the Associated Press reports is "up to 50 times more potent than heroin." The Midwest Medical Examiner's Office released its autopsy findings on Thursday, revealing that the singer had "administered the drug himself, but the date he took it was unknown," per the AP.

According to the news outlet, fentanyl-related overdoses have been on the rise in the U.S. in recent years, and the synthetic drug is frequently sought out by "pain patients who have built up a tolerance to other prescription painkillers, or who have become addicted." A drug overdose was suspected in Prince's death, and an autopsy was conducted the day after he was found at his Paisley Park studios.

The week before his death, Prince's plane made an emergency landing in Illinois after he was reportedly found unconscious on the aircraft. According to the AP, "first responders gave him a shot of Narcan, an antidote used in suspected opioid overdoses."

Now, authorities are looking into how Prince came into possession of those drugs in the first place, with two doctors at the center of the investigation. One, addiction specialist Dr. Howard Kornfeld, had reportedly been contacted by Prince's representatives on April 20, asking for help for the singer. The other, family practitioner Dr. Michael Todd Schulenberg, treated Prince on April 7 and April 20, and had prescribed him medications in the past, though specific details about those drugs were not released.

Prince, who shot to international musical fame in the '80s and starred in the classic flick "Purple Rain," was 57.

[via: The Associated Press]

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