The decision comes in the wake of 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 But the subpoena, issued in April, is no longer valid because it was contingent upon prosecutors pursuing the case, the official said Wednesday.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.
There was no immediate response to a message left with spokeswomen for the U.S. Attorney's office and Olsen's attorney, Michael C. Miller.
DEA investigators suspect the painkillers oxycodone and hydrocodone found in Ledger's system were obtained with phony prescriptions or other illegal means. Oxycodone is sold as OxyContin; and hydrocodone as Vicodin.
Earlier this week, Miller insisted the "Full House" actress had already told the government she "does not know the source of the drugs Mr. Ledger consumed."
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 was 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.
