Detectives investigating Prince’s death are now focusing on where the fatal drug came from: was it prescribed by a doctor or obtained on the black market?
Prince was found collapsed in an elevator at his Minneapolis estate in April after taking a synthetic opioid called fentanyl, which is 50 times more potent than heroin.
As little as two milligrams can be lethal.
The 57-year-old had reportedly worked for 154 hours without sleep in the days beforehand.
The AP news agency said the main line of inquiry was described by a law enforcement official speaking on condition of anonymity.
While fentanyl can be obtained legally as a painkiller on prescription, anyone found guilty of supplying it illegally could be charged with opioid distribution resulting in death.
The charge carries a mandatory minimum sentence of 20 years in prison.
Another official who spoke in August said Prince had not had a prescription in Minnesota for any controlled substances in the last 12 months.
Previous findings have indicated an illegal source.
Some pills containing fentanyl that were taken from the star’s home after he died were reportedly falsely labelled.
Traffickers in fact often pass fentanyl off under a different name.
Identifying who sent Prince the drug – or who handed it over – could be extremely tricky because of the subterfuge involved.
Some suppliers are said to take orders on the dark net, and then get the drugs delivered by unsuspecting mail order companies.
(c) Sky News 2016