LOS ANGELES (REUTERS) - Music superstar Prince, who was found dead in his home in a Minneapolis suburb in late April, died of an accidental, self-administered overdose of an opioid painkiller, the county medical examiner said in a death report on Thursday.

The Midwest Medical Examiner's Office in Anoka County, Minnesota, had been investigating the 57-year-old singer's death since he was discovered in an elevator in his home and studio on April 21.

"The decedent self-administered fentanyl," the medical examiner's report said, listing cause of death as "fentanyl toxicity" and noting it was accidental.

The investigation of Prince's death had centred on painkillers after prescription opioid medication was found at the scene and after reports he had been struggling with an addiction and been scheduled to meet with a doctor who specialises in treating dependency.

Fentanyl is a highly addictive opiate that is more powerful than morphine and is used to treat patients with severe pain, often after surgeries.

Federal authorities, including the Drug Enforcement Administration, were involved in the investigation. It took weeks for officials to release the cause of death because they were waiting for the results of toxicity tests.

Prince, whose full name was Prince Rogers Nelson, did not leave a will, and his multimillion-dollar estate including royalties from his more than 30 albums is being handled by a court.

The songwriter's hits included Purple Rain and When Doves Cry.