An initial autopsy into the death of Hollywood actor Heath Ledger was 'inconclusive', according to U.S reports. The New York City Medical Examiner carried out the autopsy on Ledger at its Manhattan base to try and determine what killed him. Police officers say that they discovered a string of prescription and non-prescription sleeping medication in the apartment where the 'Brokeback Mountain' star was found dead last night. Police insiders say they believe that Ledger's death was most probably an accidental one and not suicide. Now the Medical Examiner's office will carry out additional tests, which could take up to ten days to complete, TV station 'NBC 4' is reporting. FURTHER NEWS: Michelle Williams' mum and her Dawson's Creek co-star Busy Phillips have rushed to New York to comfort Heath Ledger's ex-lover. Michelle's mother, Carla, and Phillips, who is the godmother of Ledger and Williams' daughter Matilda, arrived at the couple's Brooklyn home in the early hours of this morning.