Actress Mary-Kate Olsen has been spared the ordeal of testifying before federal prosecutors over the death of actor Heath Ledger.

American federal prosecutors have closed their investigation into Ledger's death, saying there is 'no viable target'.

The US Attorney's office had been working with the Drug Enforcement Administration in a bid to find out how Ledger obtained two of the drugs involved in his accidental overdose.

A law enforcement source said the case has been closed 'because they don't believe there's a viable target'.

Silence: Mary Kate-Olsen has refused to answer questions about Heath Ledger's death from an accidental drug overdose in January

Mary-Kate Olsen had previously insisted she has 'nothing to do whatsoever' with the drugs found in Heath Ledger's body or his home following his death.

Reports suggested Olsen had refused to talk to federal investigators about Ledger's accidental overdose unless she was given immunity from prosecution.

The 22-year-old actress was the first person Ledger's housekeeper called after his masseuse found the actor dead in his New York City apartment.

Despite reports Olsen was not co-operating with the investigation, her lawyer Michael C. Miller says his client had already spoken to investigators.

Heath Ledger as the Joker in the Dark Knight

A statement from Miller said: 'Despite tabloid speculation, Mary-Kate Olsen had nothing whatsoever to do with the drugs found in Heath Ledger's home or his body, and she does not know where he obtained them.

'Regarding the Government's investigation, at Ms. Olsen's request, 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.'

Heath's body was found at his SoHo apartment on January 22nd this year after he overdosed on a mixture of prescription drugs and painkiller OxyContin.

Investigators believe the powerful painkiller may have been obtained illegally and are desperate to find out where it came from.