Federal officials have launched an investigation in a bid to discover how actor Heath Ledger was able to get the prescription drugs that caused his death last month.

On Wednesday, the New York Medical Examiner ruled the star died from an accidental overdose of six prescription medications, including painkillers, sleeping pills, anti-anxiety pills and antihistamines.

A statement from the Medical Examiner reads: "Mr. Heath Ledger died as the result of acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam, and doxylamine. We have concluded that the manner of death is accident, resulting from the abuse of prescription medications."

Now Drug Enforcement Agency (DEA) officials have teamed up with police to find out how Ledger managed to obtain all his prescription drugs.

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

Ledger, who was 28 when he died, is expected to be buried at his family's plot in Karrakatta Cemetery in his native Perth, Australia on Saturday, with his ex-fiancee, Michelle Williams, and their two-year-old daughter Matilda in attendance.

(This news article provided by World Entertainment News Network)