Mary-Kate Olsen by Dimitrios Kambouris/ Mary-Kate Olsen by Dimitrios Kambouris/

Federal investigators have a grand jury subpoena that could force Mary-Kate Olsen to testify in their investigation of Heath Ledger's fatal drug overdose, but are negotiating with her attorney to try to get her to talk voluntarily about how he got two of the painkillers that contributed to his death, a law enforcement official told

As first reported Monday by The New York Post, the former Full House actress has requested immunity from prosecution in return for her testimony about The Dark Knight star.

The law enforcement source, who spoke to on condition of anonymity because the investigation is ongoing, said it was unclear why Olsen is seeking immunity. Though Ledger had several legal prescriptions, the investigation is focused on how he got OxyContin and Vicodin, for which he did not have prescriptions, the official said.

"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," Olsen's attorney, Michael C. Miller, said in a statement Monday.

Tabloids have also questioned why a masseuse who found Ledger in January called Olsen before calling 911. Olsen then sent her own personal security to Ledger's apartment to help. The official said the timeline of Ledger's death was not under investigation.

The Drug Enforcement Administration acquired the subpoena on April 23 and has been negotiating since then with her attorney about her testimony, the official said.

Do you think officials should grant Olsen's request for immunity? - Tim Molloy