Ledger starred in the latest Batman film The Dark Knight
US investigators are to drop a criminal case into how Heath Ledger obtained two powerful painkillers that contributed to his death, an official has said.
The Drug Enforcement Administration (DEA) had suspected oxycodone and hydrocodone found in Ledger's system were obtained with false prescriptions.
But an official in New York told the AP news agency prosecutors now thought there was not "a viable target".
Ledger was found dead at his home in January after an accidental overdose.
Earlier this week, it was revealed that Ledger's friend, actress Mary-Kate Olsen, was demanding immunity from prosecution before she would speak to the DEA about the death and the actor's drug use.
At the time, Olsen's lawyer Michael C Miller said his client had told the authorities all she knew and that she "does not know the source of the drugs Mr Ledger consumed".
Olsen took calls from Ledger's masseuse after his body was found
The actress was the first person contacted by the masseuse who discovered Ledger's body.
The DEA had obtained a subpoena that could have forced Olsen to speak out.
But the official, who spoke on the condition of anonymity, told AP that was no longer valid because the case was being dropped.
The case could still be revived if evidence of a crime emerged, the official added.
Other potential witnesses had all 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 said.
Other drugs taken by Ledger, including sleeping pills and anti-anxiety medication, were prescribed legally by doctors in Texas and California.
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.