UNITED STATES—The death investigation for singer, Prince (who passed away in April), was completed on Thursday, June 2, showing that the musician died from an accidental overdose of an opioid called fentanyl, according to the Midwest Medical Examiner’s Office.

“The decedent self-administered fentanyl” and the box marked “Accident” is checked in the report that was released on Thursday.

Fentanyl is a powerful synthetic opiate analgesic that is similar to morphine, but more potent, according to the National Institute in Drug Abuse. The Associated Press was the first to report on the opioid overdose. According to the AP, it was not immediately clear if Prince had a prescription for the drug or not.

Prince died at the age of 57 on April 21 at his Paisley Parks estate in Minnesota. His body was found in an elevator inside his home. One week prior to his death he had made an emergency stop in Moline, Illinois when he was flying back from an Atlanta concert.

The Associated Press indicated he was found unconscious on the plane and first responders administered a shot of Narcan which is an antidote used if there is suspicion of an opioid overdose. Prince had a reputation for clean living, but longtime friend Sheila E. said Prince suffered from physical issues with his hip and knees from years of jumping off risers and speakers in heels during his performances.

If it is found that Prince had received the fentanyl illegally, the source that provided the drug to him could face criminal charges. Illegal distribution of the drug to someone who dies afterwards faces a mandatory minimum sentence of 20 years in jail in accordance with federal law. Under Minnesota Law, the person can face a third degree murder charge and up to 25 years behind bars.

In the investigation completed by the Carver County Sheriff’s Office, the U.S. Attorney’s Office in Minnesota and the Drug Enforcement Administration, two doctors names have come up. Dr. Michael Todd Schulenberg, a family practitioner, who treated Prince in the weeks before his death and prescribed medications that were not specified in the search warrant for the hospital Schulenberg is employed by.

Schulenberg treated Prince on April 7 and on April 20, the day before he died.

The other doctor is Dr. Howard Kornfield, a California addiction specialist, who was asked by Prince’s representatives to help Prince the day before he died. Kornfield flew his son, Andrew, out to Prince and he was among the people who discovered Prince’s body on April 21. Andrew Kornfield is not a doctor and was carrying buprenorphine, a medication to treat opioid addiction, with the intent of giving it to a Minnesota doctor on April 21.

The investigation could possibly be set to focus on determining who supplied the fentanyl to Prince and whether that source was legitimate or illegal. Authorities may look at Prince’s associates to help determine this.