An investigation into how Heath Ledger obtained drugs that contributed to his fatal overdose has been dropped, according to a law enforcement official.

The official said federal prosecutors had tried to find out who supplied the actor with two types of powerful painkillers but had given up “because they don’t believe there’s a viable target.”

Speaking to news agency The Associated Press on condition of anonymity, the official said prosecutors in the U.S. attorney's office in Manhattan recently conceded they were not going to find out where The Dark Knight star obtained oxycodone and hydrocodone, two of six drugs found in the 28-year-old’s system.

The official’s allegations follow reports that actress Mary-Kate Olsen, 23, had refused to answer questions over Ledger’s January overdose unless she was granted immunity from prosecution.

But the former child star’s lawyer, Michael C. Miller, insisted Olsen had nothing more to offer the investigation: “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 became famous alongside her twin sister, Ashley, when they starred in Eighties TV show Full House, was the first person to be called when a masseuse found Ledger’s body in the bedroom of his apartment in Manhattan seven months ago.

The masseuse phoned Olsen three times before calling emergency services. After the actress was called a fourth time, she sent her personal security guards to the apartment to help.

In February, New York's chief medical officer, Charles Hirsch, confirmed the father of one died of an accidental overdose of prescription drugs, including anti-anxiety medication and sleeping pills, prescribed to him by doctors, and the painkillers, investigators believe he obtained illegally.

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

Having been nominated for an Oscar for his performance in Brokeback Mountain, Ledger is tipped to win a post-hummus gong at next year’s awards for his critically-acclaimed role in the Batman blockbuster The Dark Knight.

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