Federal investigators in New York want to question actress Mary-Kate Olsen about how Heath Ledger got two powerful painkillers that contributed to his accidental overdose death, but she wants immunity or she won't talk, a law enforcement official said Monday.

Olsen's lawyer has twice refused requests to have her sit down with investigators probing the death of the scene-stealing star of The Dark Knight earlier this year in a Manhattan apartment, a report by the Associated Press said.Australian actor Heath Ledger, currently on screen as the Joker in The Dark Night, died of an accidental overdose of prescription drugs in January. (Canadian Press)

The report confirms stories that Olsen — a close friend of Ledger who learned about the 28-year-old actor's death in a frantic phone call from a masseuse — wants a promise of immunity before she speaks to the Drug Enforcement Administration. Olsen owns the apartment Ledger was found in.

The activity demonstrates that federal authorities are still actively trying to figure out how Ledger got some of the drugs that killed him.

DEA investigators suspect the painkillers oxycodone and hydrocodone found in his system were obtained with phony prescriptions or other illegal sources, the official said. Oxycodone is sold as OxyContin and hydrocodone as Vicodin.

The other drugs that resulted in what the medical examiner called "acute intoxication," including anti-anxiety and sleeping pills, were prescribed legally by doctors in California and Texas, the official said.

Authorities have obtained a subpoena that could force Olsen to appear before a grand jury if negotiations with her lawyer fail, the official said.

Other potential witnesses have answered questions voluntarily, including doctors, Ledger's ex-girlfriend Michelle Williams and people who were in his apartment around the time of his death, the official added.

There was no immediate response to a message left with a spokeswoman for Olsen.

The masseuse discovered Ledger's body on Jan. 22 after the actor had completed filming his role as the Joker in the new Batman film.

Police say the masseuse spent nine minutes making three calls to Olsen before dialing 911 for help, then called the Full House actress a fourth time after paramedics arrived.

At some point during the flurry of calls, Olsen, who was in California, summoned her personal security guards to the apartment to help, police said.

Shortly after the Australian-born actor's death, Olsen issued a statement that read: "Heath was a friend. His death is a tragic loss."