As the inquiry concerning Heath Ledger’s accidental overdose that caused the actor’s death continues, the federal Drug Enforcement Administration office in Manhattan has confirmed to “Access Hollywood” that it has issued a subpoena that would constrain Mary-Kate Olsen to testify in front of a grand jury if negotiations with her lawyer failed.

Five drugs were identified in Heath Ledger’s body at the time of his death, according to the DEA, three of which he had acquired with the help of doctors in Texas and Los Angeles. The investigation is mainly focusing on how he managed to obtain Oxycontin and Vicodin without a legal prescription.

In January this year, the 28-year-old actor who starred in blockbusters such as “Brokeback Mountain” and “The Dark Knight” was found dead in his Soho apartment. In addition to this, Mary-Kate Olsen was the first person the masseuse who discovered Heath Ledger’s body called.

According to a New York Post report, the actress broke the silence and said that she had nothing to do with the deadly overdose Ledger took. As reported by the same source, Olsen’s claims of innocence come after a paper’s statement on Monday that she had “something to hide in her personal life.” The report was triggered by her request for immunity before testifying to federal investigators.

Federal investigators want to interrogate Mary-Kate Olsen about how Heath Ledger obtained the two powerful painkillers that contributed to his death. Nevertheless, the actress’ attorney, Michael C. Miller, added that she “does not know the source of the drugs Mr. Ledger consumed.”