updated 8:16 p.m. ET Aug. 6, 2008

NEW YORK - 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 But the subpoena, issued in April, is no longer valid because it was contingent upon prosecutors pursuing the case, the official said Wednesday.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.

DEA 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.

Slide show   Heath Ledger, 1979-2008 Take a look back at the key roles and leading ladies in the actor’s life more photos

Miller insisted this week that Olsen, a former child star on the sitcom “Full House,” had already told the government she “does not know the source of the drugs Mr. Ledger consumed.”

Other potential witnesses apparently answered questions voluntarily, including doctors, Ledger’s ex-girlfriend Michelle Williams and people in his apartment around the time of his death.

Other drugs taken by Ledger, 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. It’s common for the DEA to investigate an overdose death with so many different drugs involved, a DEA spokesman said last month.

The masseuse discovered Ledger’s body on Jan. 22. Police say she spent nine minutes making three calls to Olsen before dialing 911 for help, then called the actress a fourth time after paramedics arrived. At some point during the flurry of frantic calls, Olsen, who was in California, summoned her personal security guards to the apartment to help, police said.

Ledger died after filming “The Dark Night,” the latest movie in the “Batman” series, in which he has earned rave reviews for playing a maniacal Joker. The film had taken in more than $400 million domestically as of Monday.

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