MINNESOTA (AP) — Prince died of an accidental overdose of fentanyl, a powerful opioid painkiller that is up to 50 times more potent than heroin, autopsy results released Thursday show.
The findings confirm suspicions that opioids played a role in the death of the 57-year-old superstar musician, who was found dead April 21 at his Minneapolis-area estate.
It was not immediately clear whether Prince had a prescription for the drug and, if not, how he obtained it. The results raised the possibility that anyone who provided the drug illegally could face criminal charges.
[PHOTOS: Prince through the years]
After Prince died, authorities began reviewing whether an overdose was to blame and whether he had been prescribed drugs in the preceding weeks.
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 the death investigation is complete, and it had no further comment.
Since his death, a series of reports have given insight into his drug use. The report from Minneapolis Star Tribune cited sources, detailed how those around Prince tried to get him help, but ultimately, those efforts failed. For example, Prince’s staff reached out to a physician to help treat him for an addiction to prescription pain mediation, the newspaper reported.
What is fentanyl?
Fentanyl is a synthetic drug that is partly responsible for a recent surge in overdose deaths in some parts of the country. Because of its risks, it is tightly controlled by the Food and Drug Administration.
[READ: What is fentanyl and why did it kill Prince?]
Pain patients who have built up a tolerance to other prescription painkillers, or who have become addicted, sometimes seek out stronger drugs such as heroin or fentanyl.
More than 700 fentanyl-related overdoses were reported to the Drug Enforcement Administration in late 2013 and 2014. The drug also has legitimate medical uses.
Prince's final show was in Atlanta
Prince died 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 Associated Press 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.
Prince had spoken about struggling with childhood epilepsy, and friends said he had hip trouble. His former percussionist, Sheila E., told The Associated Press that Prince suffered the effects from years of jumping off risers and speakers on stage while wearing high heels.
"There was always something kind of bothering him, as it does all of us," she said. "I hurt every single day. You know we're like athletes, we train, and we get hurt all the time. We have so many injuries."
The singer's death came two weeks after he canceled concerts at the Fox, saying he wasn't feeling well. He then played a pair of makeup shows April 14 in that city, apologizing to the crowd shortly after coming on stage. He was scheduled to perform two shows in St. Louis but canceled them shortly before his death.
How did he get the drug?
The focus of the investigation will now probably turn to determining who supplied the fentanyl and whether the sources were legitimate or illegally, said Gal Pissetzky, a Chicago-based attorney who has represented multiple clients facing drug charges. He has no link to Prince.
Authorities may also look to the singer's associates.
"The investigation may expand to people who surround him," Pissetzky said. "If fentanyl was obtained illegally, I don't think Prince would have gone out to meet someone in a dark alley to get the substance."
If a street dealer was the source, identifying that person won't be easy.
"It'll be very, very difficult," he said. "These guys don't write receipts, and they change phones all the time."
Illegally distributing fentanyl to someone who then dies from it is punishable by a mandatory minimum sentence of 20 years under federal law. Under Minnesota law, the same actions can result in third-degree murder charges and up to 25 years in prison.
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.
[READ: Prince saw doctor on day before singer's death, search warrant says]
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.
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 Kornfeld was among the people who found Prince's 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.
Information from the Associated Press