A court document says a doctor prescribed oxycodone for Prince under the name of the musician's friend to protect his privacy.
Prince was 57 when he was found alone and unresponsive in an elevator at his Paisley Park home on April 21.
Autopsy results showed he died from an accidental overdose of fentanyl, a synthetic drug 50 times more powerful than heroin.
According to search warrants unsealed Monday, authorities searched Paisley Park, cellphone records of Prince's associates, and Prince's emails to try to determine where he got the fentanyl that killed him.
The documents suggest Prince was struggling with prescription opioid addiction.
One affidavit says Dr. Michael Todd Schulenberg admitted he prescribed oxycodone for Prince in a Prince's associate's name "for Prince's privacy."
A message left with Schulenberg's attorney wasn't immediately returned.
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