NEW YORK -- Federal prosecutors have dropped the investigation into how Heath Ledger obtained two painkillers that contributed to his overdose, a law enforcement official said. Mary-Kate Olsen, who had refused to speak without immunity, will now not be forced to.

The official said prosecutors in the U.S. attorney's office in Manhattan bowed out "because they don't believe there's a viable target." The official spoke to The Associated Press on condition of anonymity because no charges have been filed.

Rebekah Carmichael, a spokeswoman for the U.S. attorney's office, said it's the office's policy to "neither confirm nor deny the existence of an investigation." There was no immediate response to a message left for Olsen's attorney, Michael C. Miller.

Drug enforcement agents had issued a subpoena to Olsen to try to get her to tell what she might know about Ledger's accidental drug overdose, law enforcement officials said. However, by closing the case, the former "Full House" star will not be forced to talk.

Olsen was subpoenaed on April 23 but would not answer questions without an offer of immunity, one source said.

The subpoena could have forced Olsen if she continued to hold out. But the subpoena is no longer valid because it was contingent upon prosecutors pursuing the case, the official said Wednesday.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.

In a statement sent to News 4 New York Monday from Michael C. Miller, on behalf of Olsen, a lack of involvement of the former "Full House" star was proclaimed in the investigation.

"Despite tabloid speculation, Mary-Kate Olsen had nothing whatsoever to do with the drugs found in Heath Ledger's home or his body, and she does not know where he obtained them," the statement stated. "Regarding the government's investigation, at Ms. Olsen’s request, we have provided the government with relevant information including facts in the chronology of events surrounding Mr. Ledger's death and the fact that Ms. Olsen does not know the source of the drugs Mr. Ledger consumed."

Read The Full Statement On Behalf Of Olsen

Authorities said Olsen was the first person called by a masseuse who found the 28-year-old "Dark Knight" actor's lifeless body in his Manhattan apartment.

Investigators said of the five drugs in Ledger's system, they still have not found legal prescriptions for two of the drugs -- OxyContin and Vicodin.

Investigators said there is no evidence of a crime but want to speak to Olsen so that they could close out the case. Doctors and others linked to the "Dark Night" star have cooperated, they said.

The masseuse discovered Ledger's body on Jan. 22 in his SoHo apartment. Police said 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.

The "Dark Knight" had taken in $400.04 million domestically as of Monday.