Court documents unsealed on Monday (17 April) showed that Prince's doctor prescribed drugs under the name of the musician's close friend and drummer, Kirk Johnson. Dr Michael Todd Schulenberg admitted prescribing the musician with the drug in the name of Johnson "for Prince's privacy".

The legal papers also suggest that Prince was battling with prescription opioid addiction. According to the search warrant, investigators were told by witnesses that Prince "recently had a history of going through withdrawals which are believed to be the result of abuse of prescription medication", reported CNN.

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A message left with Dr Schulenberg's lawyer was not returned, according to AP.

According to search warrants applied for by Detective Angela Nucci and unsealed on Monday, Paisley Park was thoroughly examined by law enforcement officers. They looked for any illicit narcotics that could explain the cause of the decedent's death.

The search also included going through phone records of Prince's associates as well as emails in order to find out where he received the drug fentanyl.

Autopsy results revealed that Prince died from an accidental overdose of the synthetic opioid drug fentanyl, which is 50 times stronger than heroin. The drug is usually used to treat patients in severe pain, according to the National Institute on Drug Abuse. High doses of opioids can cause breathing to stop completely, leading to death.

Dozens of counterfeit pills were found in a search of Paisley Park, including at least one that tested positive for fentanyl. Authorities do not know where the drugs came from and investigators have yet to interview Prince's associates or ask a grand jury to consider whether criminal charges are an option, according to an official with knowledge of the investigation.

An important witness investigators want to interview is Kirk Johnson, who is also the estate manager at Paisley Park. Johnson was also with Prince six days earlier when he fell ill on a flight home from a performance in Atlanta and had to be revived with two doses of an opioid antidote.

Johnson's attorney, F Clayton Tyler, confirmed that his client has not been subpoenaed. Prince's close friend declined to talk to AP but revealed in a CBS interview, pointing to his head: "Kirk has a vault. Right here. It's never gonna be unlocked."