Actress Mary-Kate Olsen, who asked for immunity before cooperating with the police in the investigation of Heath Ledger's death, has been legally compelled to testify in front of a grand jury in the case, Access Hollywood reported.

While the NYPD closed their case looking into the death of the 28-year-old "Dark Knight" star, the Federal Drug Enforcement Agency is trying to determine where Ledger obtained the prescription medications that led to a fatal drug interaction. Investigators from the DEA looking into how Ledger obtained the drugs concluded that he had legal prescriptions from two doctors, one in Texas and one in Los Angeles, for all medications but one of them, OxyContin.

As reported recently, Mary-Kate Olsen requested immunity from prosecution before talking to the DEA, fueling rumors she might have been involved in actor's death on January 22. Others with legal backgrounds have said that she might just be proceeding cautiously as her lawyer advises and that we should not jump to conclusions.

On Monday, Olsen's lawyer released a statement dismissing tabloid speculation saying his client "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" and that she had already provided the government with valuable information. Still, rumors persist that she knows more than she claims she does regarding who might have supplied Ledger with the drugs.

Despite her request, the actress has not been granted immunity and was served a subpoena to testify in front of a grand jury. A grand jury determines whether there is enough evidence to proceed with a trial. The proceedings are private and closed to the public, and Olsen does not have the right to have her attorney with her when she testifies. If the grand jury decides that there is enough evidence to go forward with a federal trial it could possibly be open to the public.

Access Hollywood which broke the news about the subpoena on Tuesday night, gave no timeline for when Olsen might be called to testify. The same story claims Ledger died from five medications, not six, and that there are two drugs he didn’t have prescriptions for, OxyContin and Vicodin while the official coroner’s report listed six drugs as the cause of death.

Olsen drew attention to herself as being the first person called by the masseuse who discovered Ledger’s body in his Soho apartment. The actress, who is best-known for her role as Michelle Tanner on the sitcom "Full House," sent two private security guards to Ledger's apartment but failed to tell the masseuse to call 911.

According to a legal source quoted by Access, seeking immunity is not the same as admitting guilt. "It may not mean anything other than she's getting good legal advice," defense attorney Benjamin Brafman told the show. "You don't know what the DEA may suspect in this case, and I think it's a cautious way to proceed. I think it would be wrong to assume she's done something criminal simply because she's invoking her Fifth Amendment against self-incrimination."