[Black Cobra]

US federal authorities will investigate why Heath Ledger was on a dangerous mix of six prescription drugs which they say was responsible for his death.

Medical experts have said it was unusual that Ledger could have been prescribed such a dangerous mix of drugs and some believe that it was likely he received the medication from more than one doctor.

"The US system is a very low service industry and it's not community based pharmacy like here in Australia," National President of the Pharmacy Guild of Australia, Kos Sclavos said.

"It's quite likely that he received the prescribtions from more than one doctor."

Hollywood star Heath Ledger, found dead in his New York apartment two weeks ago, died of accidental intoxication brought on by his abuse of a cocktail of prescription drugs, officials have revealed.

Ledger, who scored critical acclaim for his performance as a repressed gay cowboy in Brokeback Mountain in 2005, was taking a mixture of pain killers, sedatives and antidepressants used to treat anxiety and sleep problems.

"Mr Heath Ledger died as the result of acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam, and doxylamine," the city's chief medical examiner's office said in a statement.

"We have concluded that the manner of death is accident, resulting from the abuse of prescription medications," it added.

The cause of the 28-year-old Australian's death had been shrouded in mystery, with an initial autopsy proving inconclusive and conflicting speculation that he was a drug user and suffering from pneumonia.

He had been found naked and unresponsive in his Manhattan apartment, with police saying that prescription medications were found in the residence but that a rolled-up 20 dollar bill near the body had tested negative for drugs.

His family insisted throughout that he would not have taken his own life.

Ledger told The New York Times in November, during the shooting of the latest Batman movie, The Dark Knight, in which he portrays The Joker, that he often took Ambien, a prescription sleeping aid, to help his insomnia.

"Last week I probably slept an average of two hours a night," he told The Times. "I couldn't stop thinking. My body was exhausted and my mind was still going," he said.

He was reported to have said that taking one pill had no effect on him and that he'd take a second one to fall asleep, only to awake an hour later with his "mind still racing."

Ledger's father Kim, said he hoped the official toxicology results would end rumors surrounding his son's death and appealed for privacy.

"Today's results put an end to speculation, but our son's beautiful spirit and enduring memory will forever remain in our hearts," he said in a statement.

"While no medications were taken in excess, we learned today the combination of doctor-prescribed drugs proved lethal for our boy," he said, adding "we respectfully request the worldwide media allow us time to grieve privately."

Ledger was seen as a rising star in Hollywood and his death on January 22 sent shockwaves through the film industry. His memorial service in Los Angeles drew an A-list of mourners including Tom Cruise and wife Katie Holmes.

Arrangements for Ledger's burial in Perth have not been made public, but it is expected his family will hold a private funeral later this week at which they will bury their son in a family plot.

Ledger first came to prominence by acting as a homosexual athlete in a little-known Australian soap opera, Sweat, in 1996.

At 19, he left Sydney for Hollywood, where his talent was spotted by Mel Gibson when auditioning 500 actors for the role of his son in The Patriot — a break that led to his leading role in A Knight's Tale.

He was nominated for a best actor Oscar for his Brokeback Mountain role and had recently been working on The Imaginarium of Doctor Parnassus, directed by Terry Gilliam, which was due out next year.