The actress was a friend of Ledger
Actress Mary-Kate Olsen will not speak to investigators about Heath Ledger's death unless she is granted immunity from prosecution.
An official said the 22-year-old wants the assurance before speaking to the Drug Enforcement Administration (DEA).
Olsen was the first person contacted by the masseuse who discovered Ledger's body. Her lawyer said she has told the authorities all she knows.
Ledger was found dead at his home in January after an accidental overdose.
Federal investigators want to question Olsen about how Ledger obtained two powerful painkillers that contributed to his death.
Olsen's lawyer, Michael C Miller, added that she "does not know the source of the drugs Mr Ledger consumed".
According to the official - who spoke on condition of anonymity because the investigation is ongoing - Mr Miller has twice refused requests for her to speak with them.
Investigators are in negotiations with Olsen's lawyers and the prosecutor's office, it was added.
Ledger's ex-girlfriend Michelle Williams has answered questions voluntarily
The official said that authorities had obtained a subpoena that could force Olsen to appear before a grand jury if negotiations with her lawyer fail.
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 false prescriptions.
Police say the masseuse who discovered Ledger's body spent nine minutes making three calls to Olsen before dialling 911 for help, then rang the actress a fourth time after paramedics arrived.
At some point during the calls, Olsen, who was in California at the time, summoned her personal security guards to the apartment to help.