Unsealed court documents have revealed Prince was prescribed pain medication under a friend’s name and was in a relationship with a former contestant on The Voice, as the anniversary of his death approaches.

Prince was 57 when he was found dead in an elevator inside his Paisley Park estate in Minnesota on April 21.

An autopsy later determined the singer died from an overdose of fentanyl – a strong painkiller that is 50 times more powerful than morphine or heroin.

Court documents unsealed on Monday (US time) as part of the investigation into his death found that none of the medications in Prince’s home were prescribed to him.

The unsealed search warrants state that painkillers were found at several locations throughout the estate, with many concealed in vitamin pill bottles and envelopes.

No fentanyl, the painkiller determined to have killed Prince, was found in the complex.

Some of the pills, located in a suitcase, were prescribed to Prince’s long-time friend and former drummer Kirk Johnson.

One of the documents indicated that local doctor Michael Todd Schulenberg said he had prescribed oxycodone to the pop star under Johnson’s name for “privacy reasons”.

The documents detail that Dr Schulenberg visited Paisley Park after the singer’s body was found to “drop off test results”.

The doctor told investigators he saw Prince on April 7 and April 20, and prescribed medications. He left his job at North Memorial Medical Centre about three weeks after Prince’s death.

But Dr Schulenberg’s attorney said in a statement on Monday that the doctor denied directly prescribing Prince opioids or “any other person with the intent that they would be given to Prince”.

Witnesses told investigators Prince “recently had a history of going through withdrawals which are believed to be the result of abuse of prescription medication”, according to the search warrant.

The documents fail to uncover how Prince obtained the fentanyl that killed him.

Investigators are yet to charge anyone over his death and the circumstances that led to Prince overdosing remain a mystery.

Secret relationship

In addition to the drug revelations, the unsealed documents revealed that Prince had been in a romantic relationship with singer-songwriter Judith Hill.

The 32-year-old former The Voice contestant told authorities she had been in a relationship with Prince since autumn 2014, after email exchanges were discovered during the investigation.

Hill revealed that she would communicate with Prince by phone and email, using a Gmail address with the name “Peter Bravestrong”, a pseudonym he often used to conceal his identity while travelling, People.com reported.

She had been on the flight with Prince from Atlanta days before his death when he lost consciousness, forcing the plane to make an emergency landing at Moline, Illinois.

Hill had been selected to perform as Michael Jackson’s duet partner during his This Is It tour in 2009, but after months of rehearsals, the King of Pop died three weeks before the first concert.

Investigators were under instruction by authorities to seal any documents relating to the case “until April 17, 2017 or when a criminal case may be instituted, whichever occurs first”.

The investigation into Prince’s death remains “active”.