Mary-Kate Olsen has refused to speak to federal investigators about Heath Ledger’s death unless she gets immunity from prosecution first, the New York Post reported.

Olsen was the first person that Ledger’s housekeeper called after the actor’s masseuse found him dead from the accidental overdose in his apartment on January 22.

The Drug Enforcement Administration started to investigated whether Ledger was over-prescribed medicine shortly after his death. The autopsy revealed that Ledger had taken pain, sleep and anxiety pills that had been prescribed to him. But unfortunately the combination was lethal.

After interviewing the actor’s doctors, investigators found prescriptions for every drug so far except Oxycodone and Vicodin, two powerful painkillers that are often abused.

The team of investigators conducted several interviews concerning the unexpected death. They talked with Ledger’s ex-girlfriend Michelle Williams, his doctors, his masseuse and bodyguards, housekeepers and business associates, but they couldn’t convince Mary-Kate Olsen, the actor’s closest friend at the time, to talk with them.

The actress’ lawyer has repeatedly refused the feds’ attempts to question Olsen, but the officials eventually could obtain a grand-jury subpoena to compel the actress tell them everything she knew about the “Dark Knight” star’s behavior and his possible drug issues.

Officials complained that everyone was “very eager to help,” except Mary-Kate Olsen.
Olsen remained the final witness needed to complete the investigation.

Olsen's lawyer, Michael Miller, told the Post: “We are not going to comment on whether there is a criminal investigation.”

When the “Full House” actress received the news that Ledger was dead, she did not call authorities, but her bodyguards, who arrived at the same time with the paramedics.