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Heath Ledger, Mary-Kate Olsen

Jim Spellman/WireImage.com, Nick Harvey/WireImage.com

Guess Mary-Kate Olsen hasn't given officials all the "relevant information" they needed in the Heath Ledger case after all.

Days after a formal statement from the twin's rep proclaiming her innocence and insisting Olsen has fully cooperated with the Drug Enforcement Administration's investigation into how Ledger obtained medications sans prescription, E! News has confirmed that a subpoena with the star's name on it is ready and waiting to be delivered.

"It was dated and signed on April 23, but it has not been enforced yet," says a federal law-enforcement source, denying reports circulating earlier today that Olsen had already been served.

"We are still in negotiations. There are still negotiations with Mary-Kate's lawyer and also with our U.S. Attorney's Office, the prosecutors."

Should negotiations with Team Olsen fall through, the subpoena will be issued requiring the former Full Houser to be queried in front of a grand jury. Should she cooperate, as her attorney says she has been, there will be no need for the strong-arm tactics.

DEA officials are trying to determine where and how the late Ledger obtained prescriptions for OxyContin and Vicodin. The onetime Oscar nominee died of an accidental overdose on Jan. 22.

Olsen was the first person called by the masseuse who discovered Ledger's body, leading her to dispatch her security team to the Manhattan apartment.

Earlier this week, sources confirmed that Olsen was refusing to cooperate with the federal probe unless she was granted immunity in the case—something the feds say is not on the table.

But the news prompted a statement from Olsen's attorney.

"Despite tabloid speculation, Mary-Kate Olsen has nothing whatsoever to do with the drugs found in Heath Ledger's home or his body, and does not know where he obtained them," Michael C. Miller said Monday.

While Miller did not address the reports that Olsen was seeking immunity from the feds, he did call descriptions of the investigation "incomplete and inaccurate."

—Additional reporting by Claudia Rosenbaum