A law enforcement official says federal prosecutors have decided not to pursue a criminal case into how Heath Ledger got the powerful painkillers that contributed to his overdose death last January.

Federal prosecutors in New York had been overseeing a Drug Enforcement Administration probe into whether the painkillers found in Ledger's system were obtained illegally.

But the law enforcement source says prosecutors have bowed out because they don't think there's a -- quote -- "viable target."

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

A lawyer for the former child star says Olsen has told the government she doesn't know where Ledger got the drugs.