Police are investigating how Prince got the drug that killed him.

Law enforcement sources have told TMZ that the "illicit" possession of the drug is the focus of the investigation and both state and federal prosecutors are involved.

The website reports: "If someone illegally prescribed or filled a prescription for a patient who dies as a result of the drug, they could face third degree murder charges and up to 25 years in prison."

Prince, whose full name was Prince Rogers Nelson, was found unresponsive in an elevator at his Paisley Park home in Chanhassen, Minnesota on April 21 from a "self-administered fentanyl overdose".

It was previously claimed that Prince had been addicted to the painkiller Percocet for years because of problems with his hips and that he had to be given a "save shot" almost a week before his passing after he overdosed on the highly-addictive drug, which contains acetaminophen and oxycodone

Prince - who had used a cane to help him get around for many years - had long needed a double hip replacement but reportedly turned it down on numerous occasions but he was worried it would require a blood transfusion, which would have conflicted with his beliefs as a Jehovah's Witness.

The singer was rushed to hospital in Moline, Illinois on April 15 for the shot, which is administered to counteract the effects of opiates, after being taken ill on his private jet on the way back from a gig in Atlanta.