LOS ANGELES, California (X17online) - Actress Mary-Kate Olsen has refused to be questioned by federal investigators examining the accidental death of Heath Ledger unless she receives immunity, according to The New York Post.

The newspaper reports that the actress' attorney has repeatedly resisted attempts by the federal investigators to question his client, but investigators could obtain a grand-jury subpoena to force the actress to talk about the late actor and his possible drug use. All of the drugs in Ledger's body (and the prescription bottles present in his apartment) were legally obtained from two physicians except for OxyContin, and according to a source, investigators "are trying to ID the source of the OxyContin. Did it come from a dealer, from a friend? If he had a bottle from a friend, was it taken by someone else before police responded? That is what is trying to be determined."

Olsen and Ledger had been friends for nearly two years, and Olsen's masseuse, Diana Wolozin, was the one who discovered Ledger's body in his New York City residence. Wolozin phoned the actress before calling authorities, and Olsen called her bodyguards telling them to head to Ledger's home, which they did, around the same time as paramedics. The fact that Wolozin called the actress before she dialed 911 has raised significant questions as to what involvement, if any, Olsen had in the Oscar nominated actor's death.

Another source told the New York Post, "Everyone has been very eager to help, saying what a great guy Heath Ledger was, everyone except Mary-Kate, who has refused to speak." Olsen's lawyer, Michael Miller said, "We are not going to comment on whether there is a criminal investigation."

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