US federal authorities are to investigate how Heath Ledger was able to acquire the prescription drugs that led to his death.
It was confirmed yesterday that the Brokeback Mountain star was killed by an accidental overdose of prescription drugs.
A spokeswoman for the New York City medical examiner's office said the drugs included sleeping pills, painkilling medication and anti-anxiety tablets.
And a statement released by The Office confirmed the combination of drugs taken by the 28-year-old actor.
"Mr Heath Ledger died as the result of acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine," it read.
"We have concluded that the manner of death is accident, resulting from the abuse of prescription medications."
Erin Mulvey, a spokesperson for the New York Drug Enforcement Agency (DEA), has since told People magazine that the federal government aims to establish how the actor was able to obtain the drugs.
"We are working with the NYPD to identify any illegally prescribed drugs that may have been prescribed to [Ledger]," she added.
The star of A Knight's Tale was found dead in his New York City apartment on January 22nd, with a subsequent autopsy inconclusive as to the cause of his death.
He was discovered unconscious at his apartment in downtown Manhattan and pronounced dead at the scene, after his housekeeper had entered his apartment to alert him to the arrival of his masseuse.
Ledger's father Kim released a statement to Access Hollywood following the medical examiner's findings, saying: "We remain humble as parents and a family, among millions of people worldwide who may have suffered the tragic loss of a child. Few can understand the hollow, wrenching, and enduring agony parents silently suffer when a child predeceases them.
"Today's results put an end to speculation, but our son's beautiful spirit and enduring memory will forever remain in our hearts. While no medications were taken in excess, we learned today the combination of doctor-prescribed drugs proved lethal for our boy."
He added: "Heath's accidental death serves as a caution to the hidden dangers of combining prescription medication, even at low dosage."
Ledger's former partner Michelle Williams, mother to his two-year-old Matilda, this week issued her first statement since Ledger's death, saying: "My heart is broken."
The actress arrived in Perth, Australia - Ledger's home town - on Tuesday morning ahead of his funeral, believed to be scheduled forSaturday.