The singer Prince died of an opioid overdose, a US law enforcement official has told the Associated Press.
The official, who is close to the investigation, said this was established by tests after the singer's death in April at his Minneapolis home.
Detectives have already questioned a doctor who saw 57-year-old Prince 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.
Local officials said after a post-mortem examination that there was no reason to believe that Prince's death was suicide.
Prince 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.