The mystery surrounding the death of Heath Ledger continues with reports that the American actress Mary-Kate Olsen has refused to talk to police about the Australian actor’s overdose unless she is given immunity from prosecution.

American federal investigators want to question the 22-year-old former child star about how Ledger was able to attain two powerful painkillers that contributed to his accidental death in his New York apartment on January 22.

However, according to a report in the New York Post, Olsen’s lawyer has twice refused permission for her to speak with investigators without a promise of immunity.

Her lawyer Michael C. Miller said in a statement: “We have provided the government with relevant information including facts in the chronology of events surrounding Mr Ledger’s death, and the fact that Ms Olsen does not know the source of the drugs Mr Ledger consumed.”

Olsen, who lives in California, was the first person to be called when a masseuse found Ledger’s lifeless body in the bedroom of his apartment in Manhattan eight months ago.

The masseuse phoned Olsen three times before dialling 911 for help. After the fourth call the actress summoned her personal security guards to the apartment, where they arrived at the same times as ambulance officers, but Ledger could not be saved.

In February New York's chief medical officer, Charles Hirsch, found that Ledger had died of an accidental overdose.

“Mr Ledger died as the result of acute intoxication by the combined effects of oxycodone, hydrocodone, diazepam, temazepam, alprazolam, and doxylamine," he said. "We have concluded that the manner of death is accident, resulting from the abuse of prescription medications."

Drug Enforcement Administration investigators now suspect that the painkillers oxycodone and hydrocodone found in Ledger’s system were obtained with fake prescriptions or other illegal means.

A law enforcement source told Reuters that investigators are in negotiations with Olsen’s lawyers. "We have asked to interview her (twice) and it has not happened,” the source said.

Ledger, 28, was best known for his portrayal of a gay cowboy in the 2005 film Brokeback Mountain, a role which earned him a best actor Oscar nomination.

He is now tipped to receive a posthumous Oscar for his critcally acclaimed performance of the Joker in the latest Batman film, The Dark Knight, which is currently in cinemas and has broken box office records around the world.

He has a two-year-old daughter, Matilda, with his former fiancée, American actress Michelle Williams.

According to the Post, Williams was also interviewed by police, and was “extremely nice and co-operative.”

Mary-Kate Olsen first came to fame with her twin sister Ashley in 1987 when they starred in the American sitcom Full House when they were babies.

She has since starred in critically-panned, straight to DVD movies, but made millions through merchandising, a production company and a global tween-orientated clothing range which she runs with her sister.

It is not known how long Olsen knew Ledger, but after the Australian actor’s death she released a statement which read: “Heath was a friend. His death is a tragic loss.”

Heath Ledger, Hollywood's Dark Star, Buy the book

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