Probe into Heath Ledger's death is dropped...

Heath Ledger will be allowed to rest in peace.

Federal prosecutors are not going pursue a criminal case into how the star of "Brokeback Mountain" and "The Dark Knight" obtained the powerful painkillers that contributed to his overdose death this year, the Associated Press reports..

The decision comes after recent reports that Mary-Kate Olsen was demanding immunity before answering questions about the startling death of her close friend and his drug use. Authorities say she was the first person called by a masseuse who found Heath's lifeless body in his Manhattan apartment.

Investigators suspect the painkillers found in Ledger's system, oxycodone and hydrocodone, were obtained with phony prescriptions or other illegal means. Oxycodone is sold as OxyContin and hydrocodone as Vicodin. Other drugs taken by Heath, including anti-anxiety medication and sleeping pills, were prescribed legally by doctors in California and Texas.

The medical examiner's office wouldn't say what concentrations of each drug were found but made clear he was killed by the combination - not an excess of any one drug in particular.