Mary-Kate Olsen has refused to speak to authorities investigating the overdose that killed Oscar-nominated actor Heath Ledger in January.

The 22 year-old billionairess will not present herself to be interviewed by federal investigators probing the death of her rumored lover unless she receives immunity from prosecution, the New York Post reported Monday.

Investigators believe MK could be instrumental in helping them track down the source of the OxyContin that killed Ledger.

“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,” a source told The Post.

“Everyone has been very eager to help, saying what a great guy Heath Ledger was, everyone except Mary-Kate, who has refused to speak.”