Mary-Kate Olsen is refusing to be interviewed by federal agents investigating her friend Heath Ledger’s accidental death until she receives immunity from prosecution The New York Post has claimed. Olsen was the first person called after her masseuse discovered Heath Ledger's body in his New York apartment in January. According to sources the actress' lawyer has “repeatedly rebuffed attempts” to answer questions about the Dark Knight’s death. “Everyone has been very eager to help, saying what a great guy Heath Ledger was,” a source said. “Ms. Williams was extremely nice and cooperative. Everyone except Mary-Kate, who has refused to speak.” According to sources, all of the drugs in Ledger's body were legally obtained from doctors with the exception of the powerful painkiller OxyContin. Federal investigators are keen to know how Ledger came by it. “Investigators are trying to ID the source of the OxyContin,” a source said. “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.”















