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As the probe into Heath Ledger's death continues, federal investigators are seeking to question "Full House" alum and Ledger friend Mary-Kate Olsen, specifically about how the late actor obtained two painkillers that contributed to his accidental overdose this past January, sources tell ET.

A law enforcement source tells ET that Olsen is asking for immunity before agreeing to speak with the Drug Enforcement Agency, but authorities have obtained a subpoena that could force the actress to appear in front of a grand jury if negotiations fail.

On Monday, Olsen's lawyer released this statement:

"Despite tabloid speculation, Mary-Kate Olsen had nothing whatsoever to do with the drugs found in Heath Ledger's home or his body, and she does not know where he obtained them. Regarding the Government's investigation, at Ms. Olsen's request, we have provided the Government with relevant information including facts in the chronology of events surrounding Mr. Ledger's death and the fact that Ms. Olsen does not know the source of the drugs Mr. Ledger consumed. We don't know the source of the information being quoted in the media regarding the Government's inquiry, but these descriptions are incomplete and inaccurate."