What Does Olsen Know About Heath's Death?

Aug. 4, 2008— -- Does Mary-Kate Olsen have something to hide?

After Olsen refused to cooperate with investigators, a federal grand jury issued a subpoena as part of a criminal investigation into the source of the drugs that killed her close friend Heath Ledger, ABC News has learned.

The federal investigation into Ledger's death hit a roadblock in the form of the pint-sized actress. Sources close to the investigation said Olsen is the final witness in their investigation but has so far refused numerous requests to her lawyer Michael Miller to cooperate.

According to government sources, the subpoena was issued on April 23, but the U.S. Drug Enforcement Agency, which launched an investigation after Ledger's accidental death from a prescription drug cocktail, has not yet served Olsen with it. The agency and her lawyer are still trying to work out a deal in which the actress receives immunity in exchange for her cooperation, the sources said.

Miller, Olsen's attorney issued the following statement Monday afternoon: "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."

The United States Attorney's Office, which has the authority to grant immunity, refused to comment on an ongoing investigation.

Agents have able to determine the source for all the drugs Ledger ingested, except for one, the powerful painkiller OxyContin, which they believe was obtained illegally.

Government sources told ABC News that investigators have run down "every legitimate lead" on where Ledger might have obtained the OxyContin. Now, in light of her refusal to cooperate, investigators are focused on whether Olsen may have supplied the potent pills to Ledger.

"Maybe she did, or her MD did, maybe she had her own supply and shared it with him," one government source said.

Olsen is still considered a witness and not a suspect in the criminal probe, according to sources. But without her cooperation or until she is brought before a grand jury, the government appears unlikely to determine where the OxyContin came from.

Sources say all the other prescription drugs obtained in the United States -- from a doctor in Texas and another in LA -- were legitimately acquired.