Federal prosecutors in New York have decided to close the investigation into how actor Heath Ledger acquired two powerful painkillers that contributed to his fatal overdose, a law enforcement official announced on Wednesday, as reported by the Associated Press.

Prosecutors in the U.S. attorney’s office in Manhattan had been running a Drug Enforcement Administration inquiry into whether the drugs found in Ledger’s body were obtained without a legal prescription. However, the prosecutors have abandoned the investigation “because they don’t believe there’s a viable target,” the official informed the Associated Press on condition of anonymity because no charges have been filed.

The decision follows recent reports that actress Mary-Kate Olsen was requiring immunity before testifying about the shocking death of her close friend. According to authorities, she was the first person called by the masseuse who discovered the 28-year-old actor’s body in his Manhattan apartment.

The DEA had acquired a subpoena that would have constrained Olsen to testify if the negotiations with her attorney had failed. Nevertheless, the subpoena issued by a federal grand jury will not be enforced, the official told the AP on Wednesday, although the case could still be resumed if evidence of a crime comes to surface.

DEA investigators believe the painkillers found in Heath Ledger’s system, oxycodone and hydrocodone, had been obtained through illegal means. Other medications taken by “The Dark Knight” actor, including anti-anxiety medication and sleeping pills, were prescribed legally by physicians in California and Texas.

The masseuse found Ledger’s lifeless body on Jan. 22. According to police officials, she spent nine minutes telephoning Mary-Kate Olsen three times before dialing 911, then called the twin actress a fourth time following the paramedics’ arrival. Sometime during the telephone calls, Olsen, who was in California at that time, sent her personal security guards to the apartment to help, police said.