America’s federal Drug Enforcement Administration (DEA) has issued a subpoena to require actress Mary-Kate Olsen to testify in front of a grand jury as part of the continuing investigation into The Dark Knight star Heath Ledger’s death.

Rumors swirled on the Net earlier this week that Olsen had refused to be questioned by federal authorities unless she was guaranteed immunity from prosecution.

According to a report in the New York Post, federal authorities have been frustrated by repeated refusals from Mary-Kate Olsen to cooperate with their investigation into Brokeback Mountain star Heath Ledger’s death.

Olsen has refused to be questioned by federal authorities unless she is given immunity from prosecution, according to the Post.

Olsen was the first person called by the masseuse who found Ledger’s body in his SoHo, New York apartment in January.

According to an unnamed source quoted by the Post, everyone else federal authorities have approached to interview about the events surrounding Ledger’s overdose has cooperated fully, except Olsen.

A report posted on People.com from Olsen’s lawyer, however, refutes the information published by the New York Post.

According to attorney Michael C Miller, Olsen has provided investigators with all “relevant information” regarding Ledger’s death and has “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.”

“We don’t know the source of the information being quotes in the media regarding the government’s inquiry,” Miller said in the statement reported by People.com, “but these descriptions are incomplete and inaccurate.”

Although Ledger’s death was determined to be the result of an accidental overdose of prescription drugs, federal investigators are still trying to track down how the actor acquired some of the drugs without a legal prescription.

Five medications were found in Ledger’s body at autopsy, three of which were legally obtained, according to the DEA, but federal authorities are still trying to determine the source of the powerful drugs Oxycontin and Vicodin found during the investigation.

Federal investigators confirmed to Access Hollywood on Tuesday that a subpoena has been issued to force Olsen to testify before a grand jury about any information she may have regarding the drugs which caused Ledger’s death. Olsen’s lawyer continues to insist that the actress “does not know the source of the drugs Mr. Ledger consumed.”

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