More than five months after Prince's fatal drug overdose, investigators have narrowed their focus to two main questions.

That's according to a law enforcement official, who said the questions are whether doctors illegally prescribed opioids meant for the pop star and whether the fentanyl that killed him came from a black-market source.

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

Prince was 57 when he was found April 21 at his suburban Minneapolis estate. Authorities have revealed little publicly, saying only that their investigation is ongoing.

The law enforcement official spoke on condition of anonymity because the person was not authorized to comment.