The Drug Enforcement Administration has closed its investigation into the death of Heath Ledger.

Federal officials have dropped the investigation “because they don’t believe there’s a viable target,” an official told the Associated Press late Wednesday.

DEA officials had been trying to understand how the 28-year-old actor acquired OxyContin and Vicodin without a legal prescription.

The New York Drug Enforcement Administration Office had obtained a subpoena in an attempt to force actress Mary-Kate Olsen to testify in front of a grand jury as part of their investigation into Ledger’s drug overdose last January.