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.

The DEA is still running down leads on other drugs found in Ledger's possession that appear to have been acquired in Italy and the United Kingdom.

Olsen was the first person called after her masseuse discovered Ledger's body in his SoHo apartment in January. Instead of calling 911, Olsen reportedly phoned her bodyguards, who arrived on the scene at the same time as paramedics.

Investigators believe they have interviewed everyone connected to Ledger and his death, including the masseuse, bodyguards, doctors, housekeepers, business associates and even the mother of his 2-year-old daughter, Matilda, his "Brokeback Mountain" co-star Michelle Williams.

Olsen was not interviewed by New York police either. An NYPD spokesperson said, "Detectives interviewed all relevant witnesses, including all who were at the scene. They had all the information they needed without interviewing Olsen, who was not in New York at the time."

Erin Mulvey, a DEA spokesperson, said that in the past two years prescription drug abuse has soared, particularly among young people. According to a recent study, one in 10 students in grades 8, 10 and 12, admitted to using Viocodin and one in 20 confessed to using OxyContin.