Police investigating the death a year ago of pop star Prince found numerous opioids scattered around his home but appear not to have identified where or who supplied the dose of fentanyl that caused his death, according to court documents unsealed today.

Some of the strong painkillers found at the musician's Paisley Park complex outside Minneapolis had prescriptions in the name of his friend and bodyguard, the affidavits and search warrants showed.

The probe included searches of Prince's computer, mobile phone records of his friends and interviews with his associates. In October 2016 it was termed "an active homicide investigation" in the documents, but no one has been criminally charged.

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Prince, 57, was found dead at the complex on April 21, 2016. The official cause of death was given as an accidental, self-administered overdose of the painkiller fentanyl.

No prescriptions were found for fentanyl - a powerful drug that is 50 times stronger than heroin. The documents were kept sealed until Monday because Carver County, Minnesota, prosecutors feared potential witnesses might flee or potential evidence be destroyed.

According to the search warrants, investigators found several pills labelled Watson 853 - the identifier for generic hydrocodone-acetaminophen.

A search also turned up other "numerous narcotic controlled substance pills" in various containers, including vitamin bottles, some of which were prescribed to the musician's bodyguard.

"Many of those areas where the pills were located would be places Prince would frequent, such as his bedroom and wardrobe/laundry room," one document said.

Detectives were "made aware by witnesses that were interviewed, that Prince recently had a history of going through withdrawals, which are believed to be the result of the abuse of prescription medication," the documents said.

The unsealed documents confirmed reports from law enforcement sources last year that multiple prescription painkillers were found in Prince's home, belying his public reputation for living a clean and healthy vegan lifestyle.

The documents showed that some of the prescriptions were made out in the name of Kirk Johnson, Prince's bodyguard, to safeguard "Prince's privacy."

Johnson's lawyer did not return calls for comment on Monday.

The documents also revealed that Prince did not use a cellphone, and that he had email accounts in various aliases. He also did not have a regular doctor and his team would arrange for various physicians to give him vitamin B-12 shots before performances.