Minneapolis (AP) -- Word from authorities that Prince died of an overdose of fentanyl, a powerful opioid that is up to 50 times more potent than heroin, is far from the end of the investigation. In some ways, it's just the beginning.
After Thursday's announcement about the superstar's death, investigators in the months to come will try to determine whether the singer had a prescription for the drug or whether it was supplied illegally. If it's the latter, someone could face criminal charges carrying years, or even decades, in prison.
Prince was found dead April 21 at his Minneapolis-area estate, and at least one friend has said he suffered from intense knee and hip pain from many years of performances.
Although the death was formally ruled an accident, that merely signified that it was not intentional and does not preclude a criminal prosecution.
According to a one-page report released by a medical examiner, Prince administered the drug himself on an unknown date. The office said the death investigation is complete, and it had no further comment.
Confirmation that Prince died of an opioid overdose was first reported by The Associated Press. The autopsy report was released hours later.
Fentanyl is a synthetic painkiller. Patients who have built up a tolerance to other prescription painkillers sometimes seek it out, and it 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, but much of it is manufactured illegally.
Kent Bailey, head of the DEA in Minneapolis, said the agency will continue investigating along with Carver County authorities and the U.S. Attorney's Office. He declined to offer details, but said "rest assured, we will be thorough."
Legal experts say the focus of the investigation will now probably turn to whether the source or sources of the fentanyl were legal or not. Often, such investigations include grand jury subpoenas for records or for testimony from individuals.
Authorities may also look to the singer's associates.
"The investigation may expand to people who surround him," said Gal Pissetzky, a Chicago-based attorney who has represented multiple clients facing drug charges who has no link to Prince. "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.
Prince, 57, 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 he was found unconscious on the plane, and first responders gave him a shot of Narcan, an antidote used in suspected opioid overdoses.
The names of at least two doctors have come up in the death investigation.
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, according to a search warrant that did not specify which medications.
Schulenberg saw Prince April 7 and April 20 — the day before his death — according to the warrant. Schulenberg's attorney, Amy Conners, said the doctor was interviewed by investigators April 21, right after Prince's death, but has had "no further requests from investigators" since. She declined to comment further.
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 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 a medication that can be used to treat opioid addiction, 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.
On Thursday, Mauzy said his clients never delivered, dispensed or administered any medication to Prince. The Kornfelds "were simply trying to help," he said.
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, and then came the emergency landing in Moline. He canceled two shows in St. Louis shortly before his death.
The superstar had a reputation for clean living, and some friends said they never saw any sign of drug use.
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.
Tarm reported from Chicago. Follow him on Twitter at http://twitter.com/mtarm
Associated Press Medical Writer Carla K. Johnson in Chicago also contributed to this report.
Prince autopsy report: www.anokacounty.us/DocumentCenter/View/10066