NEW YORK (AP) -- Federal investigators want to question Mary-Kate Olsen about how Heath Ledger got two powerful painkillers that contributed to his accidental overdose death, but she's refusing to talk without immunity, a law enforcement official said Monday.

Olsen's lawyer has twice refused requests for her to speak with investigators, said the official, who spoke on condition of anonymity because the investigation was ongoing. The lawyer, Michael C. Miller, said the "Full House" actress has nothing to do with the drugs, and has already told the government everything she knows.

"We have provided the government with relevant information including facts in the chronology of events surrounding Mr. Ledger's death," Miller said in a statement Monday, "and the fact that Ms. Olsen does not know the source of the drugs Mr. Ledger consumed."

The official confirmed a report that Olsen wants a promise of immunity from prosecution before speaking to the Drug Enforcement Administration. Olsen was a close friend of Ledger's, and was the first person called by a masseuse who found the 28-year-old "Dark Knight" actor's lifeless body in his Manhattan apartment.

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 all have answered questions voluntarily, including doctors, Ledger's ex-girlfriend Michelle Williams and anyone who was in his apartment around the time of his death, the official added.

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

The other drugs, including anti-anxiety medication and sleeping pills, were prescribed legally by doctors in California and Texas, the official said.

The medical examiner's office wouldn't say what concentrations of each drug was found, but made clear he was killed by the combination - not an excess of any one drug in particular. It's common for the DEA to investigate an overdose death with so many different drugs involved, spokesman Garrison Courtney said last month.

The DEA's New York office declined comment on the stalemate with Olsen, which was first reported Monday by the New York Post. There was no immediate response to a message left with a spokeswoman for Olsen.

The masseuse discovered Ledger's body on Jan. 22. Police say she spent nine minutes making three calls to Olsen before dialing 911 for help, then called the actress a fourth time after paramedics arrived. At some point during the flurry of frantic 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."