America's Drug Enforcement Agency and the NYPD have launched an official investigation into Heath Ledger’s death.

The New York Medical Examiner ruled yesterday that the Brokeback Mountain star died from an accidental overdose of six prescription medications, including painkillers, sleeping pills, anti-anxiety pills and antihistamines.

The DEA and police will now endeavour to find out how Ledger was able to get the prescription drugs that caused his death last month.

DEA spokesman Erin Mulvey says, "We are working with the NYPD to identify any illegally prescribed drugs that may have been prescribed to Ledger."

As reported by Entertainmentwise yesterday, the 28-year-old is expected to be buried at his family's plot in Karrakatta Cemetery in his native Perth, Australia.

A private funeral will reportedly be held on Saturday with his ex-fiancee Michelle Williams and their two-year-old daughter Matilda in attendance.