NEW YORK (AP) — Federal prosecutors have decided not to pursue a criminal case into how Heath Ledger obtained the powerful painkillers that contributed to his overdose death this year, a law enforcement official said Wednesday.

Prosecutors in the U.S. attorney’s office in Manhattan had been overseeing a Drug Enforcement Administration probe into whether the painkillers found in Ledger’s system were obtained illegally. But the prosecutors have bowed out “because they don’t believe there’s a viable target,” said the official, who spoke on condition of anonymity because no charges have been filed.

The decision comes after recent reports that actress 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 the 28-year-old “Dark Knight” actor’s lifeless body in his Manhattan apartment.

The DEA had obtained a subpoena that could have forced Olsen if she continued to hold out. But the subpoena, issued in April, is no longer valid because it was contingent upon prosecutors pursuing the case, the official said Wednesday. The official added that the case could still be revived if evidence of a crime emerges.

Rebekah Carmichael, a spokeswoman for the U.S. attorney’s office, said it’s the office’s policy to “neither confirm nor deny the existance of an investigation.” There was no immediate response to a message left for Olsen’s attorney, Michael C. Miller.