Mary-Kate Olsen has refused to talk to federal investigators regarding Heath Ledger's death in January unless she gets immunity from prosecution first, according to the New York Post. The actress was the first person Ledger's housekeeper called after his masseuse found the actor dead from an accidental overdose in his SoHo apartment January 22.

A lawyer for Olsen has "repeatedly rebuffed attempts by the feds" to speak to his client, the paper reported. Investigators are hoping that Olsen, who was allegedly dating the actor around the time of his death, can help with a few details, like where he got the drugs that caused the overdose and the events that led up to it.

Federal agents have identified the two doctors who prescribed Ledger the drugs that were found in his home at the time of his death, however, they still don't know how he obtained OxyContin, a powerful painkiller that showed up in the actor's autopsy. New York Police Department officials closed their case without talking to Olsen, but the U.S. Drug Enforcement Administration reportedly wants to speak to her regarding her involvement with the actor.

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

Federal investigators have already talked to a slew of people connected to "The Dark Knight" star, including his doctors, the masseuse, his family and actress Michelle Williams, who is the mother of his two-year-old daughter, Matilda.