Ledger (left) and Mary-Kate Olsen Photo by: Jeff Vespa / WireImage; Cliff Lipson / Showtime

Mary-Kate Olsen wants immunity from prosecution before she will cooperate with a federal investigation into close friend Heath Ledger's accidental overdose death, PEOPLE has confirmed.

Investigators could subpoena the 22-year-old actress but hope to work this out, a law enforcement source said Monday.

Olsen's publicist and lawyer didn't immediately comment.

Ledger, 28, was found in the bedroom of his New York apartment Jan. 22 by his housekeeper and a masseuse who had arrived for an appointment with the actor. The masseuse called Olsen before calling 911, police said.

The Drug Enforcement Administration's probe into whether Ledger was over-prescribed medicine began shortly after his death. An autopsy revealed he had prescription pain, anxiety and sleep medications in his system and the combination killed him.

Investigators have interviewed Ledger's doctors and found prescriptions for every drug so far except Oxycodone and Vicodin, two powerful and often-abused painkillers, a source said.

Federal investigators conducted interviews, including one of Ledger's ex-girlfriend, Michelle Williams, the source said.

The only thing person left to interview is Olsen, the source said.

"We are also still waiting for information from pharmacy purchases by Ledger in Italy and England," the source said.

Olsen's desire for immunity was first reported by the New York Post.

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