MINNEAPOLIS (AP) — More than five months after Prince’s fatal drug overdose, investigators have narrowed their focus to two main questions: whether doctors illegally prescribed opioids meant for the pop star and whether the fentanyl that killed him came from a black-market source, a law enforcement official said.

Those lines of inquiry raise the prospect that a doctor or doctors could be charged with writing unlawful prescriptions and that a separate suspect or set of suspects with ties to narcotics trafficking could be charged with supplying the fatal dose.

Prince was 57 when he was found April 21 in an elevator at his suburban Minneapolis studio and estate. Authorities have revealed little publicly about their investigation, saying only that the probe is ongoing.

The law enforcement official who described the investigation has knowledge of the inquiry but spoke on condition of anonymity because the person was not authorized to comment on the record. The person declined to provide any additional details.

Investigations of fatal overdoses can be lengthy and complex, especially when drug traffickers or other underworld figures are involved.

Ryan Pacyga, a Minneapolis criminal defense attorney who is not connected to the Prince case, said law enforcement is not going to rush unless there is a risk to the public or immediate danger to others.

In typical drug cases, investigators will subpoena documents including computer files, emails and financial records. When looking at where the fentanyl came from, they will “follow the money” and look at orders, shipments and the bank accounts or credit cards that made payments, Pacyga said.

They will also identify people they want to speak with.

“They’ll interview from the bottom up,” Pacyga said. “They’ll talk to the people who really don’t have much criminal exposure and keep working their way up to who they think the big wrongdoers are.”

Notable drugs cases are often prosecuted in federal, not state, court.

Among the most serious federal charges a person who supplied the fentanyl could face – be it a doctor, trafficker or friend – is opioid distribution resulting in death, which carries a mandatory minimum of 20 years in prison.