Mary-Kate Olsen is refusing to cooperate with an investigation into Heath Ledger’s fatal drug overdose unless she is granted immunity from prosecution, said the New York Post this week. Olsen was the first person Ledger’s masseuse contacted when she discovered the actor’s body in his apartment in January; reports at the time indicated that Olsen then dispatched her personal bodyguards to the scene. The Drug Enforcement Administration is trying to determine where Ledger got OxyContin, the only drug in his system for which he did not have a prescription.