Heath Ledger by Kevin Mazur/WireImage.com Heath Ledger by Kevin Mazur/WireImage.com

Federal prosecutors have dropped their plan to pursue an inquiry into Heath Ledger's death.

Prosecutors had been working with the DEA on their investigation into how Ledger obtained the painkillers that led to his overdose, and if they were acquired illegally. According to the AP, a law enforcement official said Wednesday that the probe was dropped "because they don't believe there's a viable target." The source remained anonymous because no charges had been filed.

The decision comes on the heels of Mary-Kate Olsen's request for immunity before testifying about Ledger's death. The actress asked for legal protection in exchange for speaking voluntarily to authorities. She was the first person called - even before 911 - by masseuse Diana Wolozin when Ledger's body was found on Jan. 22, and has since only issued a short statement about his passing that read, "Heath was a friend. His death is a tragic loss."

Olsen's lawyer said in a statement Monday that she "had nothing to do with the drugs," OxyContin and Vicodin, that were found in the Dark Knight star's home after he died. He did not have prescriptions for either drug, according to an official who spoke anonymously to TVGuide.com. The DEA had obtained a subpoena for Olsen in their preparation for an inquiry, but their legal mandate is invalid now that the case has been abandoned.

Did the authorities make the right call? - Anna Dimond