MINNEAPOLIS — More than five months after Prince’s fatal drug overdose, a law enforcement source says 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.

Those questions 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 drug.

Prince was 57 when he was found April 21 in an elevator at his suburban Minneapolis estate. Authorities have revealed little publicly about their investigation. The law enforcement official who described the investigation has knowledge of the inquiry but spoke on condition of anonymity.