Prince had an "exceedingly high" level of fentanyl in his body when he died, say experts, citing a toxicology report from his post-mortem examination.
The US pop star was 57 when he was found alone and unresponsive in a lift at his Paisley Park home in Minnesota on 21 April, 2016.
Weeks after his death, medical examiners revealed he died of an accidental overdose of fentanyl, a synthetic opioid 50 times more powerful than heroin.
Now a confidential toxicology report provides more information about just how much fentanyl was in his system.
It says the concentration of the drug in his blood was 67.8 micrograms per litre, adding there have been fatalities among people with blood levels ranging from three to 58 micrograms per litre.
The report also says the level of fentanyl in his liver was 450 micrograms per kilogram, and points out liver concentrations greater than 69 micrograms per kilogram "seem to represent overdose or fatal toxicity cases".
Experts who are not connected to the Prince investigation said the data leaves no doubt that fentanyl killed him.
Dr Lewis Nelson, chairman of emergency medicine at Rutgers New Jersey Medical School, said: "The amount in his blood is exceedingly high, even for somebody who is a chronic pain patient on fentanyl patches."
He called the fentanyl concentrations "a pretty clear smoking gun".
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There was also what experts called a potentially lethal amount of fentanyl in Prince's stomach.
Dr Charles McKay, president of the American College of Medical Toxicology, said the findings suggest Prince took the drug orally, while fentanyl in the blood and liver suggest it had some time to circulate before he died.