Mary-Kate Olsen

Possibly shady?

Mary-Kate Olsenhas refused to be interviewed by federal investigators from the U.S. Drug Enforcement Agency who are probing Heath Ledger’s accidental drug-related death. She is lawyered up and won’t talk unless she receives immunity from prosecution, which is her constitutional right.

Why does the 4-foot-11, 90-pound Olsen need immunity?

The New York Post is reporting that MK’s lawyer, Michael Miller, has repeatedly rebuffed the feds’ attempts to talk to Olsen, who was called by her masseuse after the woman discovered Ledger’s unresponsive body in his SoHo apartment in January 2008.

Everyone involved with Ledger — doctors, the masseuse, bodyguards, housekeepers, even Michelle Williams, the mother of his baby daughter, Matilda — has already cooperated and been questioned.

Olsen is reportedly the last witness needed to conclude the investigation into where Heath got the drugs and medicines that caused his accidental death.

The cause of death was determined to be an overdose of sleep-aid medications, prescribed by two doctors, and of a powerful painkiller, OxyContin, from an unknown source.

Investigators want to find out where Heath got the OxyContin.

If Mary-Kate can help investigators get to the bottom of this, shouldn’t she help?

Asked whether he asked for immunity for his client, Mary-Kate Olsen’s lawyer told the Post, “We are not going to comment on whether there is a criminal investigation,” and declined to speak further.