Mary Kate Olsen has been subpoenaed to testify in front of a Grand Jury as investigators hope to end the mystery surrounding Heath Ledger's drug overdose death. Drug Enforcement Administration (DEA) officials in New York are calling on Olsen, Ledger's close friend, to fill in the blanks related to the six-month-long investigation, according to U.S. news show Access Hollywood.

The latest news comes a day after her lawyer claimed the actress has provided law officials with all the information she knows about Ledger's accidental drug overdose in January.

The 22-year-old actress' testimony is sought after because the masseuse who found Ledger dead called Olsen before contacting the emergency services.

Reports on Monday suggested Olsen was refusing to co-operate with the federal probe until she was guaranteed immunity from prosecution.

But her lawyer Michael C. Miller insisted, "We don't know the source of the information being quoted in the media regarding the Government's inquiry, but these descriptions are incomplete and inaccurate."

A timeline for Olsen's testimony has not been released.

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