Pint-size actress Mary-Kate Olsen has refused to be interviewed by federal investigators probing the accidental drug death of her close friend Heath Ledger unless she receives immunity from prosecution, The Post has learned.

The actress’ lawyer has repeatedly rebuffed attempts by the feds to question Olsen, who was the first person called after her masseuse discovered Ledger’s body in his SoHo apartment in January.

Frustrated federal officials could obtain a grand-jury subpoena to compel the funky “Full House” actress to tell them whatever she knows about the “Dark Knight” star’s behavior, his possible drug use and the events of that fateful morning, according to sources.

Olsen’s lawyer, Michael C. Miller, today denied that she was hiding any information.

“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,” he said in a statement.

Miller added that “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.

“We don’t know the source of the information being quoted in the media regarding the government’s inquiry, but these descriptions are incomplete and inaccurate.”

Probers have interviewed everyone connected to Ledger and his death, including his doctors, the masseuse, bodyguards, housekeepers, business associates and even the mother of his 2-year-old daughter, Matilda, his “Brokeback Mountain” co-star Michelle Williams.

“Ms. Williams was extremely nice and cooperative,” a source said.

Another added, “Everyone has been very eager to help, saying what a great guy Heath Ledger was, everyone except Mary-Kate, who has refused to speak.”

Jan. 26: A Free Pass for Mary-Kate

Jan. 27: Mary-Kate’s Team First at Apt.

Jan. 27: Four Days Later, Olsen Keeps Her Silence

That source explained that Olsen would be the final witness they need to conclude their investigation into where he got his drugs and medicines.

Ledger – whose performance as the Joker in “The Dark Knight” has ignited Oscar buzz – died of a potent cocktail of prescription drugs and OxyContin, the latter of which was likely obtained illegally.

His death further confirms speculation that OxyContin abuse continues to soar among celebrities and non-celebrities alike.

His death was ruled an accidental drug overdose by the medical examiner.

And NYPD investigators closed their probe without interviewing Olsen because, they said, they got all the cooperation they needed from others. That decision, however, surprised some observers, considering Olsen’s role as a potential witness.

Instead of calling emergency responders after getting the call from her masseuse, Olsen telephoned her bodyguards in the Big Apple, telling them to race to Ledger’s Broome Street home, where they arrived just as paramedics got to the scene.

The US Drug Enforcement Administration, which opened an investigation to determine the sources of Ledger’s drugs, was not as sanguine as the NYPD about talking to the 4-foot-11, 90-pound Olsen.

The feds contacted her lawyer, Michael Miller, several times recently seeking her cooperation, but Miller has told the feds Olsen won’t be interviewed unless she gets immunity, exercising her constitutional rights.

According to sources, all of the drugs in Ledger’s body and discovered nearby in prescription bottles were legally obtained from two physicians – with the exception of OxyContin, a powerful painkiller.

Investigators “are trying to ID the source of the OxyContin,” a source said. “Did it come from a dealer, from a friend? If he had a bottle from a friend, was it taken by someone else before police responded? That is what is trying to be determined.”

The DEA probers were also aggressive with the NYPD, the sources said, and were forced to threaten to obtain a grand-jury subpoena before getting the department’s Ledger files.

An NYPD spokesman said the department never balked at handing over the files and did so after receiving a routine federal administrative subpoena.

Asked whether he asked for immunity for his client, Olsen’s lawyer said, “We are not going to comment on whether there is a criminal investigation,” and declined to speak further.

The sources said Ledger’s employees reached out to Olsen because they panicked at finding his body and did not know what to do.

As for the bodyguards, they claimed they responded only to offer help.

– Murray Weiss