Prince died of accidental painkiller overdose

Investigators have been reviewing the cause of singers death

Updated 21.15

Musician and singer Prince died of an accidental overdose of the powerful painkiller Fentanyl, it has been revealed.

A one-page report by the Midwest Medical Examiner's Office in the US says the 57-year-old administered the drug himself, but when he took it was unknown.

The office said it has completed its investigation and had no further comment to make.

Prince was found dead at his estate in the Minneapolis area on April 21st.

Fentanyl is a synthetic opioid - 50 times more potent than heroin - that has been partly responsible for a recent surge in overdose deaths in some parts of the America. It also has legitimate medical uses.

The findings confirm suspicions that opioids played a role in the star's death. 

Afterwards authorities began reviewing whether an overdose was to blame and whether he had been prescribed drugs in the preceding weeks.

Prince's death came less than a week after his plane made an emergency stop in Illinois, so he could undergo medical treatment. He was found unconscious on the aircraft as he was returning from an Atlanta concert.

First responders gave him a shot of 'Narcan', an antidote used in suspected opioid overdoses.

At least two doctors' names have come up in the death investigation by the Carver County Sheriff's Office, the US Attorney's Office in Minnesota and the Drug Enforcement Administration.

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 Dr Schulenberg at the time.

Dr Schulenberg saw Prince on April 7th and April 20th - the day before his death - according to the warrant. The doctor's lawyer has declined to comment on the case.

Dr Howard Kornfeld, a California addiction specialist, was asked by Prince's representatives on April 20th to help the singer, and sent his son Andrew on a red-eye flight that night.

Andrew was among the people who found Prince's unresponsive body the next morning, according to Dr Kornfeld's lawyer, William Mauzy.

The younger Kornfeld was carrying buprenorphine, a medication that can be used to treat opioid addiction.

Mauzy explained that Andrew Kornfeld intended to give the medication to a Minnesota doctor who was seeing Prince on April 21st. Mauzy has refused to identify that doctor.

Prince's death came two weeks after he cancelled concerts in Atlanta, saying he wasn't feeling well.

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 Associated Press that Prince had physical issues from performing. She cited hip and knee problems that she said came from years of jumping off stage speakers in heels.