Mary-Kate Olsen was never interviewed by police regarding her involvement in the death of actor Heath Ledger, despite the fact that Ledger's masseuse bizarrely alerted Olsen of Ledger's lifeless body before she called the police, and the fact that Olsen's bodyguards may have arrived at Ledger's apartment three minutes before paramedics did. Now, the New York Post reports that federal drug agents are trying to get Olsen to cooperate in their investigation into how Ledger managed to finagle a bottle of the painkiller Oxycotin—also known as "hillbilly heroin"—but that Olsen is refusing to talk unless she's granted immunity from prosecution.