New York City's Chief Medical Examiner says it's still unclear what killed actor Philip Seymour Hoffman.

A spokesperson for the medical examiner's office said on Wednesday autopsy results are inconclusive and further tests are needed.

No timetable was given as to when those results will be in.

Hoffman was found dead in his Manhattan apartment on Sunday from an apparent heroin overdose.

Police say he had a syringe in his arm and investigators found at least 50 bags of heroin in the apartment.

On Tuesday, police arrested four people on drug charges during a raid at a New York City apartment building.

They're being questioned in connection with narcotics found in Hoffman's home.