Detectives questioned the doctor who saw the 57-year-old twice in the weeks before he died. Prescription painkillers were in the singer's possession following his death, officials told US media in May.
A police warrant has also revealed that Dr Michael Schulenberg prescribed medication to the singer on 20 April - the day before he died. The warrant does not say what was prescribed or whether Prince took the drugs.
According to the autopsy report, Prince self-administered fentanyl, an opioid many times more powerful than heroin. He was found unresponsive in the elevator of his Paisley Park Studios on the morning of 21 April. First responders tried to revive him with CPR but he was pronounced dead shortly afterwards.
Artists from around the world and Prince's numerous fans paid tributes to the star.
Prince was cremated in a private ceremony on 24 April. The singer's family are understood to be planning to stage a public memorial in August. He was a prolific writer and performer from a young age, reportedly writing his first song when he was seven.
A singer, songwriter, arranger and multi-instrumentalist, Prince recorded more than 30 albums. His best known hits include Let's Go Crazy and When Doves Cry.